rtc: stk17ta8: let the core handle invalid time

Returning a valid time when the time is invalid is a bad practice, because
then userspace is not able to react on the information. Also, it doesn't
make sense to return epoch because it is already the default time.

Signed-off-by: Alexandre Belloni <[e-mail]>

drivers/rtc/rtc-stk17ta8.c

···
 	/* year is 1900 + tm->tm_year */
 	tm->tm_year = bcd2bin(year) + bcd2bin(century) * 100 - 1900;
 
-	if (rtc_valid_tm(tm) < 0) {
-		dev_err(dev, "retrieved date/time is not valid.\n");
-		rtc_time_to_tm(0, tm);
-	}
 	return 0;
 }