rtc: rv8803: Remove the check for valid time

The RTC core always calls rtc_valid_tm() after ->read_time() in case of
success (in __rtc_read_time()), so do not call it twice.

Signed-off-by: Benoît Thébaudeau <benoit@wsystem.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>