rtc: fix alarm read and set offset

[ Upstream commit fd6792bb ]

The offset needs to be added after reading the alarm value.

It also needs to be subtracted after the now < alarm test.

Tested-by: Jon Hunter <jonathanh@nvidia.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Sasha Levin <alexander.levin@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>