rtc: max77686: Fix the returned value in case of error in 'max77686_rtc_read_time()'

In case of error, we return 0.
This is spurious and not consistent with the other functions of the driver.
Commit e115a2bf has modified more than what is said in the commit
message. Reverse part of it znd return an error when needed, as it was
previously.

Fixes: e115a2bf ("rtc: max77686: stop validating rtc_time in .read_time")
Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Reviewed-by: Chanwoo Choi <cw00.choi@samsung.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>