tty/serial: Migrate dz 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: "Maciej W. Rozycki" <macro@linux-mips.org>
Signed-off-by: Dmitry Safonov <dima@arista.com>
Link: https://lore.kernel.org/r/20191213000657.931618-16-dima@arista.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/serial/dz.c

@@ -29,10 +29,6 @@
 
 #undef DEBUG_DZ
 
-#if defined(CONFIG_SERIAL_DZ_CONSOLE) && defined(CONFIG_MAGIC_SYSRQ)
-#define SUPPORT_SYSRQ
-#endif
-
 #include <linux/bitops.h>
 #include <linux/compiler.h>
 #include <linux/console.h>
@@ -787,6 +783,7 @@ static void __init dz_init_ports(void)
 		uport->ops	= &dz_ops;
 		uport->line	= line;
 		uport->mapbase	= base;
+		uport->has_sysrq = IS_ENABLED(CONFIG_SERIAL_DZ_CONSOLE);
 	}
 }