alarm : Fix the race conditions in alarm-dev.c

There will be race conditions between alarm set and alarm clear if
set_power_on_alarm is out of the lock.But set_power_on_alarm can't
be put into spin_lock. So add it into mutex_lock.

CRs-Fixed: 639115
Change-Id: I1db4836f67174b756c7f9d231f7a39cf0377d2c2
Signed-off-by: Xiaocheng Li <lix@codeaurora.org>
1 file changed