rtc: pcf8563: fix write of invalid bits to ST2 reg

The NXP datasheet says:
 "Bits labeled as N should always be written with logic 0."

At least one of those bits is sometime read as a 1, therfore violating
this rule.  To fix this we mask away those bits.

Signed-off-by: Jan Kardell <jan.kardell@telliq.com>
Cc: Alessandro Zummo <a.zummo@towertech.it>
Cc: Grant Likely <grant.likely@linaro.org>
Cc: Rob Herring <robh+dt@kernel.org>
Cc: Vincent Donnefort <vdonnefort@gmail.com>
Cc: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>