RTC: Release mutex in error path of rtc_alarm_irq_enable

On hardware that doesn't support alarm interrupts, rtc_alarm_irq_enable
could return without releasing the ops_lock mutex.

This was introduced in
	aa0be0f (RTC: Propagate error handling via rtc_timer_enqueue properly)

This patch corrects the issue by only returning once the mutex is
released.

[john.stultz: Reworded the commit log]

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: John Stultz <john.stultz@linaro.org>