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" <[e-mail]>
Signed-off-by: Dmitry Safonov <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

drivers/tty/serial/dz.c

···
 
 #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>
···
 		uport->ops	= &dz_ops;
 		uport->line	= line;
 		uport->mapbase	= base;
+		uport->has_sysrq = IS_ENABLED(CONFIG_SERIAL_DZ_CONSOLE);
 	}
 }