powerpc/io: Avoid clang null pointer arithmetic warnings

[ Upstream commit 03c0f2c2b2220fc9cf8785cd7b61d3e71e24a366 ]

With -Wextra clang warns about pointer arithmetic using a null pointer.
When building with CONFIG_PCI=n, that triggers a warning in the IO
accessors, eg:

  In file included from linux/arch/powerpc/include/asm/io.h:672:
  linux/arch/powerpc/include/asm/io-defs.h:23:1: warning: performing pointer arithmetic on a null pointer has undefined behavior [-Wnull-pointer-arithmetic]
     23 | DEF_PCI_AC_RET(inb, u8, (unsigned long port), (port), pio, port)
        | ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  ...
  linux/arch/powerpc/include/asm/io.h:591:53: note: expanded from macro '__do_inb'
    591 | #define __do_inb(port)          readb((PCI_IO_ADDR)_IO_BASE + port);
        |                                       ~~~~~~~~~~~~~~~~~~~~~ ^

That is because when CONFIG_PCI=n, _IO_BASE is defined as 0.

Although _IO_BASE is defined as plain 0, the cast (PCI_IO_ADDR) converts
it to void * before the addition with port happens.

Instead the addition can be done first, and then the cast. The resulting
value will be the same, but avoids the warning, and also avoids void
pointer arithmetic which is apparently non-standard.

Reported-by: Naresh Kamboju <naresh.kamboju@linaro.org>
Closes: https://lore.kernel.org/all/CA+G9fYtEh8zmq8k8wE-8RZwW-Qr927RLTn+KqGnq1F=ptaaNsA@mail.gmail.com
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20240503075619.394467-1-mpe@ellerman.id.au
Signed-off-by: Sasha Levin <sashal@kernel.org>

arch/powerpc/include/asm/io.h

@@ -556,12 +556,12 @@ __do_out_asm(_rec_outl, "stwbrx")
 #define __do_inw(port)		_rec_inw(port)
 #define __do_inl(port)		_rec_inl(port)
 #else /* CONFIG_PPC32 */
-#define __do_outb(val, port)	writeb(val,(PCI_IO_ADDR)_IO_BASE+port);
-#define __do_outw(val, port)	writew(val,(PCI_IO_ADDR)_IO_BASE+port);
-#define __do_outl(val, port)	writel(val,(PCI_IO_ADDR)_IO_BASE+port);
-#define __do_inb(port)		readb((PCI_IO_ADDR)_IO_BASE + port);
-#define __do_inw(port)		readw((PCI_IO_ADDR)_IO_BASE + port);
-#define __do_inl(port)		readl((PCI_IO_ADDR)_IO_BASE + port);
+#define __do_outb(val, port)	writeb(val,(PCI_IO_ADDR)(_IO_BASE+port));
+#define __do_outw(val, port)	writew(val,(PCI_IO_ADDR)(_IO_BASE+port));
+#define __do_outl(val, port)	writel(val,(PCI_IO_ADDR)(_IO_BASE+port));
+#define __do_inb(port)		readb((PCI_IO_ADDR)(_IO_BASE + port));
+#define __do_inw(port)		readw((PCI_IO_ADDR)(_IO_BASE + port));
+#define __do_inl(port)		readl((PCI_IO_ADDR)(_IO_BASE + port));
 #endif /* !CONFIG_PPC32 */
 
 #ifdef CONFIG_EEH
@@ -577,12 +577,12 @@ __do_out_asm(_rec_outl, "stwbrx")
 #define __do_writesw(a, b, n)	_outsw(PCI_FIX_ADDR(a),(b),(n))
 #define __do_writesl(a, b, n)	_outsl(PCI_FIX_ADDR(a),(b),(n))
 
-#define __do_insb(p, b, n)	readsb((PCI_IO_ADDR)_IO_BASE+(p), (b), (n))
-#define __do_insw(p, b, n)	readsw((PCI_IO_ADDR)_IO_BASE+(p), (b), (n))
-#define __do_insl(p, b, n)	readsl((PCI_IO_ADDR)_IO_BASE+(p), (b), (n))
-#define __do_outsb(p, b, n)	writesb((PCI_IO_ADDR)_IO_BASE+(p),(b),(n))
-#define __do_outsw(p, b, n)	writesw((PCI_IO_ADDR)_IO_BASE+(p),(b),(n))
-#define __do_outsl(p, b, n)	writesl((PCI_IO_ADDR)_IO_BASE+(p),(b),(n))
+#define __do_insb(p, b, n)	readsb((PCI_IO_ADDR)(_IO_BASE+(p)), (b), (n))
+#define __do_insw(p, b, n)	readsw((PCI_IO_ADDR)(_IO_BASE+(p)), (b), (n))
+#define __do_insl(p, b, n)	readsl((PCI_IO_ADDR)(_IO_BASE+(p)), (b), (n))
+#define __do_outsb(p, b, n)	writesb((PCI_IO_ADDR)(_IO_BASE+(p)),(b),(n))
+#define __do_outsw(p, b, n)	writesw((PCI_IO_ADDR)(_IO_BASE+(p)),(b),(n))
+#define __do_outsl(p, b, n)	writesl((PCI_IO_ADDR)(_IO_BASE+(p)),(b),(n))
 
 #define __do_memset_io(addr, c, n)	\
 				_memset_io(PCI_FIX_ADDR(addr), c, n)