If RTC time have been altered by low voltage, we notify users
that RTC time is invalid by returning -EINVAL.
The RTC time needs to be set correctly to clear the invalid flag.
Signed-off-by: NFabien Lahoudere <fabien.lahoudere@collabora.co.uk>
Signed-off-by: NAlexandre Belloni <alexandre.belloni@free-electrons.com>