tty/serial: Migrate fsl_linflexuart 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/fsl_linflexuart.c

···
  * Copyright 2017-2019 NXP
  */
 
-#if defined(CONFIG_SERIAL_FSL_LINFLEXUART_CONSOLE) && \
-	defined(CONFIG_MAGIC_SYSRQ)
-#define SUPPORT_SYSRQ
-#endif
-
 #include <linux/console.h>
 #include <linux/io.h>
 #include <linux/irq.h>
···
 		if (brk) {
 			uart_handle_break(sport);
 		} else {
-#ifdef SUPPORT_SYSRQ
 			if (uart_handle_sysrq_char(sport, (unsigned char)rx))
 				continue;
-#endif
 			tty_insert_flip_char(port, rx, flg);
 		}
 	}
···
 	sport->irq = platform_get_irq(pdev, 0);
 	sport->ops = &linflex_pops;
 	sport->flags = UPF_BOOT_AUTOCONF;
+	sport->has_sysrq = IS_ENABLED(CONFIG_SERIAL_FSL_LINFLEXUART_CONSOLE);
 
 	linflex_ports[sport->line] = sport;