tty/serial: Migrate vr41xx_siu 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/20191213000657.931618-51-dima@arista.comSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/serial/vr41xx_siu.c

@@ -7,10 +7,6 @@
  *  Based on drivers/serial/8250.c, by Russell King.
  */
 
-#if defined(CONFIG_SERIAL_VR41XX_CONSOLE) && defined(CONFIG_MAGIC_SYSRQ)
-#define SUPPORT_SYSRQ
-#endif
-
 #include <linux/console.h>
 #include <linux/errno.h>
 #include <linux/init.h>
@@ -869,6 +865,7 @@ static int siu_probe(struct platform_device *dev)
 		port = &siu_uart_ports[i];
 		port->ops = &siu_uart_ops;
 		port->dev = &dev->dev;
+		port->has_sysrq = IS_ENABLED(CONFIG_SERIAL_VR41XX_CONSOLE);
 
 		retval = uart_add_one_port(&siu_uart_driver, port);
 		if (retval < 0) {