rtc: use rtc_valid_tm() error code when reading date/time

There's a wrong comment in some RTC drivers that say it's better to ignore
rtc_valid_tm() when reading RTC timestamp. However this is wrong and is
better to return to the userspace the error if timestamp is not valid.

Signed-off-by: Andrea Scian <andrea.scian@dave.eu>
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>