rtc: ensure rtc_set_alarm fails when alarms are not supported

When using RTC_ALM_SET or RTC_WKALM_SET with rtc_wkalrm.enabled not set,
rtc_timer_enqueue() is not called and rtc_set_alarm() may succeed but the
subsequent RTC_AIE_ON ioctl will fail. RTC_ALM_READ would also fail in that
case.

Ensure rtc_set_alarm() fails when alarms are not supported to avoid letting
programs think the alarms are working for a particular RTC when they are
not.
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>

drivers/rtc/interface.c

@@ -441,6 +441,11 @@ int rtc_set_alarm(struct rtc_device *rtc, struct rtc_wkalrm *alarm)
 {
 	int err;
 
+	if (!rtc->ops)
+		return -ENODEV;
+	else if (!rtc->ops->set_alarm)
+		return -EINVAL;
+
 	err = rtc_valid_tm(&alarm->time);
 	if (err != 0)
 		return err;