tty/serial: Migrate serial_txx9 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/20191213000657.931618-41-dima@arista.comSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/serial/serial_txx9.c

@@ -12,10 +12,6 @@
  *  Serial driver for TX3927/TX4927/TX4925/TX4938 internal SIO controller
  */
 
-#if defined(CONFIG_SERIAL_TXX9_CONSOLE) && defined(CONFIG_MAGIC_SYSRQ)
-#define SUPPORT_SYSRQ
-#endif
-
 #include <linux/module.h>
 #include <linux/ioport.h>
 #include <linux/init.h>
@@ -1095,6 +1091,7 @@ static int serial_txx9_probe(struct platform_device *dev)
 		port.flags	= p->flags;
 		port.mapbase	= p->mapbase;
 		port.dev	= &dev->dev;
+		port.has_sysrq	= IS_ENABLED(CONFIG_SERIAL_TXX9_CONSOLE);
 		ret = serial_txx9_register_port(&port);
 		if (ret < 0) {
 			dev_err(&dev->dev, "unable to register port at index %d "