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
	aa0be0f4 (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>

drivers/rtc/interface.c

@@ -209,9 +209,8 @@ int rtc_alarm_irq_enable(struct rtc_device *rtc, unsigned int enabled)
 	}
 
 	if (err)
-		return err;
-
-	if (!rtc->ops)
+		/* nothing */;
+	else if (!rtc->ops)
 		err = -ENODEV;
 	else if (!rtc->ops->alarm_irq_enable)
 		err = -EINVAL;