rtc-isl12022: properly handle military hour format

Mask out PM flag when reading the hour, always set MIL bit when
writing the hour.

Signed-off-by: Roman Fietze <roman.fietze@telemotive.de>
Acked-by: Wan ZongShun <mcuos.com@gmail.com>
Cc: Alessandro Zummo <a.zummo@towertech.it>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>