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: I4226a95e499211c0d50ff7ce269467a57a410dc7
Signed-off-by: Mao Jinlong <c_jmao@codeaurora.org>