tty/serial: Migrate pmac_zilog to use has_sysrq

The SUPPORT_SYSRQ ifdeffery is not nice as:
- May create misunderstanding about sizeof(struct uart_port) between
  different objects
- Prevents moving functions from serial_core.h
- Reduces readability (well, it's ifdeffery - it's hard to follow)

In order to remove SUPPORT_SYSRQ, has_sysrq variable has been added.
Initialise it in driver's probe and remove ifdeffery.

Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: Michael Ellerman <mpe@ellerman.id.au>
Cc: Paul Mackerras <paulus@samba.org>
Cc: linuxppc-dev@lists.ozlabs.org
Signed-off-by: Dmitry Safonov <dima@arista.com>
Link: https://lore.kernel.org/r/20191213000657.931618-32-dima@arista.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/serial/pmac_zilog.c

@@ -61,10 +61,6 @@
 #define of_machine_is_compatible(x) (0)
 #endif
 
-#if defined (CONFIG_SERIAL_PMACZILOG_CONSOLE) && defined(CONFIG_MAGIC_SYSRQ)
-#define SUPPORT_SYSRQ
-#endif
-
 #include <linux/serial.h>
 #include <linux/serial_core.h>
 
@@ -1721,6 +1717,7 @@ static int __init pmz_init_port(struct uart_pmac_port *uap)
 	uap->control_reg   = uap->port.membase;
 	uap->data_reg      = uap->control_reg + 4;
 	uap->port_type     = 0;
+	uap->port.has_sysrq = IS_ENABLED(CONFIG_SERIAL_PMACZILOG_CONSOLE);
 
 	pmz_convert_to_zs(uap, CS8, 0, 9600);