tty/serial: Migrate stm32-usart 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.

Cc: Alexandre Torgue <[e-mail]>
Cc: Maxime Coquelin <[e-mail]>
Cc: [e-mail]
Cc: [e-mail]
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/stm32-usart.c

···
  * Inspired by st-asc.c from STMicroelectronics (c)
  */
 
-#if defined(CONFIG_SERIAL_STM32_CONSOLE) && defined(CONFIG_MAGIC_SYSRQ)
-#define SUPPORT_SYSRQ
-#endif
-
 #include <linux/clk.h>
 #include <linux/console.h>
 #include <linux/delay.h>
···
 	port->ops	= &stm32_uart_ops;
 	port->dev	= &pdev->dev;
 	port->fifosize	= stm32port->info->cfg.fifosize;
+	port->has_sysrq = IS_ENABLED(CONFIG_SERIAL_STM32_CONSOLE);
 
 	ret = platform_get_irq(pdev, 0);
 	if (ret <= 0)