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

commit 72fca4a4b32dc778b5b885c3498700e42b610d49 upstream.

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>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>