alarm : Change disable irq into cancel alarm

If disable alarm irq when exception occurs during setting power off
alarm, other rtc timer may be disabled. rtc_timer_cancel should be
used to ensure that rtc timer of rtc device is canceled.

CRs-Fixed: 2070501
Change-Id: I8569757df41996618effeccbfae95e0986a4fb30
Signed-off-by: Mao Jinlong <c_jmao@codeaurora.org>