UBX: Require TIMEUTC_VALIDWKN instead of TIMEUTC_VALID_UTC

This flag was frequently not set and stopped the GPSTime object
updating, and thus the HomeLocation from updating.  This seems
to be related to leap seconds but probably isn't an issue:

http://lists.linuxtogo.org/pipermail/smartphones-standards/2008-September/000440.html
Merge pull request #424 from peabody124/utc

UBX: Require TIMEUTC_VALIDWKN instead of TIMEUTC_VALID_UTC