drivers/rtc/rtc-isl12057.c: fix masking of register values

When Intersil ISL12057 support was added by commit 70e12337 ("rtc: Add
support for Intersil ISL12057 I2C RTC chip"), two masks for time registers
values imported from the device were either wrong or omitted, leading to
additional bits from those registers to impact read values:

 - mask for hour register value when reading it in AM/PM mode. As
   AM/PM mode is not the usual mode used by the driver, this error
   would only have an impact on an externally configured RTC hour
   later read by the driver.
 - mask for month value. The lack of masking would provide an
   erroneous value if century bit is set.

This patch fixes those two masks.

Fixes: 70e12337 ("rtc: Add support for Intersil ISL12057 I2C RTC chip")
Signed-off-by: Arnaud Ebalard <arno@natisbad.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Alessandro Zummo <a.zummo@towertech.it>
Cc: Peter Huewe <peter.huewe@infineon.com>
Cc: Linus Walleij <linus.walleij@linaro.org>
Cc: Thierry Reding <treding@nvidia.com>
Cc: Mark Brown <broonie@kernel.org>
Cc: Grant Likely <grant.likely@linaro.org>
Acked-by: Uwe Kleine-König <uwe@kleine-koenig.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>