RTC: fix typo in drivers/rtc/rtc-at91sam9.c

The member of the rtc_class_ops struct is called alarm_irq_enable and
not alarm_irq_enabled

CC: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Jelle Martijn Kok <jmkok@youcom.nl>
Signed-off-by: John Stultz <john.stultz@linaro.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>