drivers/rtc/rtc-isl1208.c: call rtc_update_irq() from the alarm irq handler

Previously the alarm event was not propagated into the RTC subsystem.
By adding a call to rtc_update_irq, this fixes a timeout problem with
the hwclock utility.

Signed-off-by: Jan Luebbe <jlu@pengutronix.de>
Cc: Alessandro Zummo <a.zummo@towertech.it>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>