nmea: make sure date is valid

nmea: make sure date is valid

GPS time without date was previously used to set system date:
Tue Oct 10 11:48:21 2000 user.info kernel: [  108.786639] ugps: system time differs from GPS time by more than 5 seconds. Using 2000-10-10T10:48:21 UTC as the new time
Tue Oct 10 11:49:27 2000 user.info kernel: [  174.794699] ugps: system time differs from GPS time by more than 5 seconds. Using 2020-05-26T10:49:27 UTC as the new time

Fix this by ignoring incomplete dates and wait for complete time
information before adjusting system date/time.

Signed-off-by: Daniel Golle <daniel@makrotopia.org>