tty/serial: Migrate st-asc 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: Patrice Chotard <patrice.chotard@st.com>
Cc: linux-arm-kernel@lists.infradead.org
Signed-off-by: Dmitry Safonov <dima@arista.com>
Link: https://lore.kernel.org/r/20191213000657.931618-44-dima@arista.comSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/serial/st-asc.c

@@ -5,10 +5,6 @@
  * Copyright (C) 2003-2013 STMicroelectronics (R&D) Limited
  */
 
-#if defined(CONFIG_SERIAL_ST_ASC_CONSOLE) && defined(CONFIG_MAGIC_SYSRQ)
-#define SUPPORT_SYSRQ
-#endif
-
 #include <linux/module.h>
 #include <linux/serial.h>
 #include <linux/console.h>
@@ -730,6 +726,7 @@ static int asc_init_port(struct asc_port *ascport,
 	port->fifosize	= ASC_FIFO_SIZE;
 	port->dev	= &pdev->dev;
 	port->irq	= platform_get_irq(pdev, 0);
+	port->has_sysrq = IS_ENABLED(CONFIG_SERIAL_ST_ASC_CONSOLE);
 
 	res = platform_get_resource(pdev, IORESOURCE_MEM, 0);
 	port->membase = devm_ioremap_resource(&pdev->dev, res);