rtc: rs5c372: Fix reading from rtc when the oscillator got interrupted.

When the oscillator of the rtc gets interrupted,
e.g. due to an empty battery, reading from the rtc will now return an error
and the oscillator bit will be cleared, once the rtc is successfully reset.

Signed-off-by: Oliver Rohe <oliver.rohe@wago.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>