RTC: add missing "return 0" in new alarm func for rtc-bfin.c

The new bfin_rtc_alarm_irq_enable function forgot to add a "return 0" to
the end leading to the build warning:
	drivers/rtc/rtc-bfin.c: In function 'bfin_rtc_alarm_irq_enable':
	drivers/rtc/rtc-bfin.c:253: warning: control reaches end of non-void function

CC: stable@kernel.org
CC: Thomas Gleixner <tglx@linutronix.de>
CC: Alessandro Zummo <a.zummo@towertech.it>
Signed-off-by: Mike Frysinger <vapier@gentoo.org>
Signed-off-by: John Stultz <john.stultz@linaro.org>