rtc: pcf2127: use OFS flag to detect unreliable date and warn the user

The PCF2127 datasheet states that it's wrong to say that the date in
unreliable if BLF (battery low flag) is set but instead, OSF (seconds
register) should be used to check if oscillator, for any reason, stopped.
Battery may be low (usually below 2V5 threshold) but the date may be anyway
correct (typically date is unreliable when input voltage is below 1V2).

Signed-off-by: Andrea Scian <andrea.scian@dave.eu>
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>