RTC: twl6030: correct usage of static register while reading RTC

RTC: twl6030: correct usage of static register while reading RTC

RTC stores time and date in several registers. Due to the fact that
these registers can't be read momentary there is a chance that reading
from counting registers gives an error of one minute, one hour,
one day, etc. To address the issue the RTC has hardware that can copy
the counting registers to static shadowed registers. However, while
re-using driver code for twl6030, the feature wasn't correctly
implemented. So wrong time was read one - two times per 300000 readings.
This patch corrects usage of RTC registers for twl6030.

Change-Id: Iab1a9afd381e2a3de3b6b381ebf9bb8b798a6e18
Signed-off-by: Konstantin Shlyakhovoy <x0155534@ti.com>