tty/serial: Migrate milbeaut_usio 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/milbeaut_usio.c

···
  * Copyright (C) 2018 Socionext Inc.
  */
 
-#if defined(CONFIG_SERIAL_MILBEAUT_USIO_CONSOLE) && defined(CONFIG_MAGIC_SYSRQ)
-#define SUPPORT_SYSRQ
-#endif
-
 #include <linux/clk.h>
 #include <linux/console.h>
 #include <linux/module.h>
···
 	port->irq = mlb_usio_irq[index][RX];
 	port->uartclk = clk_get_rate(clk);
 	port->fifosize = 128;
+	port->has_sysrq = IS_ENABLED(CONFIG_SERIAL_MILBEAUT_USIO_CONSOLE);
 	port->iotype = UPIO_MEM32;
 	port->flags = UPF_BOOT_AUTOCONF | UPF_SPD_VHI;
 	port->line = index;