rtc: ds1307: rx8130: honor Voltage Loss Flag when reading the time

When voltage dropped since the RTC was last set the reported time is not
reliable. In this case return an error indicator instead of a bogus
time.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>