rtc: fix rtc_time64_to_tm for 3477

The current correction for leap years will fail in 3477. 3476-12-31 being
3477-01-00 because this is 366 leap years after 1970 and 3477 isn't a leap
year.

Fix that by looping over until days is positive or zero.

Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>