rtc: rv3029: correctly handle PON and VLOW2

In case the data is invalid (PON or VLOW2 are set in STATUS, explicitly
tell userspace that the time is invalid. Only remove VLOW2 when setting a
new valid time.

Link: https://lore.kernel.org/r/20191214221022.622482-12-alexandre.belloni@bootlin.com


Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>