rtc: cmos: avoid UIP when writing alarm time

rtc: cmos: avoid UIP when writing alarm time

[ Upstream commit cd17420ebea580c22dd3a93f7237de3d2cfafc37 ]

Some Intel chipsets disconnect the time and date RTC registers when the
clock update is in progress: during this time reads may return bogus
values and writes fail silently. This includes the RTC alarm registers.
[1]

cmos_set_alarm() did not take account for that, fix it.

[1] 7th Generation Intel ® Processor Family I/O for U/Y Platforms [...]
Datasheet, Volume 1 of 2 (Intel's Document Number: 334658-006)
Page 208
https://www.intel.com/content/dam/www/public/us/en/documents/datasheets/7th-and-8th-gen-core-family-mobile-u-y-processor-lines-i-o-datasheet-vol-1.pdf
        "If a RAM read from the ten time and date bytes is attempted
        during an update cycle, the value read do not necessarily
        represent the true contents of those locations. Any RAM writes
        under the same conditions are ignored."

Signed-off-by: Mateusz Jończyk <mat.jonczyk@o2.pl>
Cc: Alessandro Zummo <a.zummo@towertech.it>
Cc: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Link: https://lore.kernel.org/r/20211210200131.153887-10-mat.jonczyk@o2.pl
Signed-off-by: Sasha Levin <sashal@kernel.org>