rtc: pcf85063: Fix reading alarm

If the alarms are disabled the topmost bit (AEN_*) is set in the alarm
registers. This is also interpreted in BCD number leading to this warning:
rtc rtc0: invalid alarm value: 2022-09-21T80:80:80

Fix this by masking alarm enabling and reserved bits.

Fixes: 05cb3a56

 ("rtc: pcf85063: add alarm support")
Signed-off-by: Alexander Stein <alexander.stein@ew.tq-group.com>
Link: https://lore.kernel.org/r/20220921074141.3903104-1-alexander.stein@ew.tq-group.com
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>