rtc: rv8803: return meaningful value for RTC_VL_READ

RV8803_FLAG_V1F means the voltage is too low to keep the temperature
compensation running and the accuracy of the RTC is affected.

RV8803_FLAG_V2F means the voltage dropped so low that data is now invalid.

Link: https://lore.kernel.org/r/20191214220259.621996-16-alexandre.belloni@bootlin.com


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