rtc: sc27xx: stop validating rtc_time in .read_time

rtc_time64_to_tm never generates an invalid tm. It is not necessary to
validate it. Also, the RTC core is always calling rtc_valid_tm after the
read_time callback.

Reviewed-by: Baolin Wang <baolin.wang@linaro.org>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>