tty/serial: Migrate sunsab 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: "David S. Miller" <[e-mail]>
Cc: [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/sunsab.c

···
 #include <asm/prom.h>
 #include <asm/setup.h>
 
-#if defined(CONFIG_SERIAL_SUNSAB_CONSOLE) && defined(CONFIG_MAGIC_SYSRQ)
-#define SUPPORT_SYSRQ
-#endif
-
 #include <linux/serial_core.h>
 #include <linux/sunserialcore.h>
 
···
 
 	up->port.fifosize = SAB82532_XMIT_FIFO_SIZE;
 	up->port.iotype = UPIO_MEM;
+	up->port.has_sysrq = IS_ENABLED(CONFIG_SERIAL_SUNSAB_CONSOLE);
 
 	writeb(SAB82532_IPC_IC_ACT_LOW, &up->regs->w.ipc);