tty/serial: Migrate atmel_serial 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.

While at it, remove forward-declaration of atmel_console - it wasn't
needed even at the moment the driver was first time introduced:
commit 1e6c9c2878c9 ("[ARM] 3242/2: AT91RM9200 support for 2.6 (Serial)")

Cc: Alexandre Belloni <[e-mail]>
Cc: Ludovic Desroches <[e-mail]>
Cc: Richard Genoud <[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/atmel_serial.c

···
 #define ATMEL_RTS_HIGH_OFFSET	16
 #define ATMEL_RTS_LOW_OFFSET	20
 
-#if defined(CONFIG_SERIAL_ATMEL_CONSOLE) && defined(CONFIG_MAGIC_SYSRQ)
-#define SUPPORT_SYSRQ
-#endif
-
 #include <linux/serial_core.h>
 
 #include "serial_mctrl_gpio.h"
···
 
 static struct atmel_uart_port atmel_ports[ATMEL_MAX_UART];
 static DECLARE_BITMAP(atmel_ports_in_use, ATMEL_MAX_UART);
-
-#ifdef SUPPORT_SYSRQ
-static struct console atmel_console;
-#endif
 
 #if defined(CONFIG_OF)
 static const struct of_device_id atmel_serial_dt_ids[] = {
···
 	atmel_port = &atmel_ports[ret];
 	atmel_port->backup_imr = 0;
 	atmel_port->uart.line = ret;
+	atmel_port->uart.has_sysrq = IS_ENABLED(CONFIG_SERIAL_ATMEL_CONSOLE);
 	atmel_serial_probe_fifos(atmel_port, pdev);
 
 	atomic_set(&atmel_port->tasklet_shutdown, 0);