tty/serial: Migrate mux 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 <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

drivers/tty/serial/mux.c

···
 #include <asm/irq.h>
 #include <asm/parisc-device.h>
 
-#if defined(CONFIG_SERIAL_MUX_CONSOLE) && defined(CONFIG_MAGIC_SYSRQ)
 #include <linux/sysrq.h>
-#define SUPPORT_SYSRQ
-#endif
-
 #include <linux/serial_core.h>
 
 #define MUX_OFFSET 0x800
···
 		port->ops	= &mux_pops;
 		port->flags	= UPF_BOOT_AUTOCONF;
 		port->line	= port_cnt;
+		port->has_sysrq = IS_ENABLED(CONFIG_SERIAL_MUX_CONSOLE);
 
 		/* The port->timeout needs to match what is present in
 		 * uart_wait_until_sent in serial_core.c.  Otherwise