Fix the logic while writing new date/time to the chip.  The driver
incorrectly wrote back register values to different registers and even
with wrong mask.  The patch adds clearing of the VLF register, which
should be cleared if all date/time values are set.

Signed-off-by: Rudolf Marek <rudolf.marek@sysgo.com>
Acked-by: Wan ZongShun <mcuos.com@gmail.com>
Cc: Martyn Welch <martyn.welch@gefanuc.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>