rtc: bq32k: Use correct mask name for 'minutes' register.

The BQ32K_SECONDS_MASK and BQ32K_MINUTES_MASK both has the same
value. This is no functional change.

Signed-off-by: Jan Östlund <jao@hms.se>
Signed-off-by: Daniel Romell <daro@hms.se>
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>