Stop enabling the oscillator and removing the oscillator failure flags in
probe. Instead, return -EINVAL in .read_time when the oscillaotr is not
start or when it failed at some point. The oscillator gets enabled on the
first .set_time after failure and the failure flags are cleared.

This also removes the possibility of an infinite loop at probe where a
failing RTC will make the goto read_rtc to be taken every time.

Tested on mcp79411.
Reported-by: Mastro Gippo <gipmad@gmail.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>