pcf85063: Set RTC device to work in 24h mode

Sometimes the device did not set default hour mode setting correctly,
which lead to conversion errors. Explicitly set device to work in 24h
mode by clearing flag in register, instead of allowing defaults.

Reviewed by: imp
Obtained from: Semihalf
Sponsored by: Alstom Group
Differential revision: https://reviews.freebsd.org/D33497