tty/serial: Migrate zs 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/20191213000657.931618-54-dima@arista.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/serial/zs.c

@@ -44,10 +44,6 @@
  * complicated and prevents the use of some automatic modes of operation.
  */
 
-#if defined(CONFIG_SERIAL_ZS_CONSOLE) && defined(CONFIG_MAGIC_SYSRQ)
-#define SUPPORT_SYSRQ
-#endif
-
 #include <linux/bug.h>
 #include <linux/console.h>
 #include <linux/delay.h>
@@ -1106,6 +1102,7 @@ static int __init zs_probe_sccs(void)
 			zport->scc	= &zs_sccs[chip];
 			zport->clk_mode	= 16;
 
+			uport->has_sysrq = IS_ENABLED(CONFIG_SERIAL_ZS_CONSOLE);
 			uport->irq	= zs_parms.irq[chip];
 			uport->uartclk	= ZS_CLOCK;
 			uport->fifosize	= 1;