drivers/serial/atmel_serial.c: fix compile when CONFIG_SERIAL_ATMEL=Y and CONFIG_SERIAL_ATMEL_CONSOLE=N

When SERIAL_ATMEL_CONSOLE is disabled, ATMEL_CONSOLE_DEVICE is set to
NULL, and trying to access ATMEL_CONSOLE_DEVICE->flags in
atmel_serial_probe makes the compile fail.  This fixes the issue by only
accessing it if CONFIG_SERIAL_ATMEL_CONSOLE is defined

Signed-off-by: Albin Tonnerre <albin.tonnerre@free-electrons.com>
Signed-off-by: Haavard Skinnemoen <haavard.skinnemoen@atmel.com>
Cc: Nicolas Ferre <nicolas.ferre@atmel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>