ds3232->mutex is used to protect for alarm operations which
need to access status and control registers.

But we can use rtc->ops_lock instead.  rtc->ops_lock is held when most
of rtc_class_ops methods are called, so we only need to explicitly
acquire it from irq handler in order to protect form concurrent
accesses.
Signed-off-by: NAkinobu Mita <akinobu.mita@gmail.com>
Signed-off-by: NAlexandre Belloni <alexandre.belloni@free-electrons.com>