rtc: ds1347: properly handle oscillator failures

rtc: ds1347: properly handle oscillator failures

The comment in the probe function stating that it disables oscillator stop
detection and glitch filtering is incorrect as it sets bits 3 and 4 while
it should be setting 5 and 6 to achieve that. Then, it is safe to assume
that the oscillator failure detection is actually enabled.

Properly handle oscillator failures by returning -EINVAL when the time and
date are know to be incorrect and reset the flag when the time is set.

Link: https://lore.kernel.org/r/20191007134724.15505-8-alexandre.belloni@bootlin.com
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>