tty/serial: Migrate sb1250-duart 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.
Signed-off-by: Dmitry Safonov <dima@arista.com>
Link: https://lore.kernel.org/r/20191213000657.931618-39-dima@arista.comSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/serial/sb1250-duart.c

@@ -15,10 +15,6 @@
  *	"BCM1250/BCM1125/BCM1125H User Manual", Broadcom Corporation
  */
 
-#if defined(CONFIG_SERIAL_SB1250_DUART_CONSOLE) && defined(CONFIG_MAGIC_SYSRQ)
-#define SUPPORT_SYSRQ
-#endif
-
 #include <linux/compiler.h>
 #include <linux/console.h>
 #include <linux/delay.h>
@@ -813,6 +809,7 @@ static void __init sbd_probe_duarts(void)
 			uport->ops	= &sbd_ops;
 			uport->line	= line;
 			uport->mapbase	= SBD_CHANREGS(line);
+			uport->has_sysrq = IS_ENABLED(CONFIG_SERIAL_SB1250_DUART_CONSOLE);
 		}
 	}
 }