rtc: rx8025: clear RTC_FEATURE_ALARM when alarm are not supported

Clear RTC_FEATURE_ALARM to signal alarms are not supported to the core
instead of checking client->irq.

Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Link: https://lore.kernel.org/r/20211107225458.111068-10-alexandre.belloni@bootlin.com