should use 'i' instead of 'd' in minmea_parse_gbs

[xiaoxiang781216]

please reference the definition here: https://gpsd.gitlab.io/gpsd/NMEA.html#_gbs_gps_satellite_fault_detection

[kosma]

Cherry-picked into master. I have also added a better unit test:

https://github.com/kosma/minmea/commit/1ac0ec3ae6f04f4fa47ed341471133132eadcf28

Thanks again for the contribution!