rtc: mt6397: fix alarm register overwrite

rtc: mt6397: fix alarm register overwrite

Alarm registers high byte was reserved for other functions.
This add mask in alarm registers operation functions.
This also fix error condition in interrupt handler.

Fixes: fc2979118f3f ("rtc: mediatek: Add MT6397 RTC driver")

Signed-off-by: Ran Bi <ran.bi@mediatek.com>
Signed-off-by: Hsin-Hsiung Wang <hsin-hsiung.wang@mediatek.com>
Link: https://lore.kernel.org/r/1576057435-3561-6-git-send-email-hsin-hsiung.wang@mediatek.com
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>