mfd: bd70528: Fix hour register mask

When RTC is used in 24H mode (and it is by this driver) the maximum
hour value is 24 in BCD. This occupies bits [5:0] - which means
correct mask for HOUR register is 0x3f not 0x1f. Fix the mask

Fixes: 32a4a4eb

 ("rtc: bd70528: Initial support for ROHM bd70528 RTC")

Signed-off-by: Matti Vaittinen <matti.vaittinen@fi.rohmeurope.com>
Acked-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>