Update VTG parsing for NMEA0183 version 2.3 and later

[SivertHavso]

See #1 (comment)

I tried to replicate the fix in RMC.HPP and RMC.CPP for the same issue (FAA mode indicator). I've also added a missing assignment for the = operator in RMC.CPP.

Example sentence NMEA0183 ver 2.3 and later:

$GNVTG,348.78,T,,M,0.03,N,0.05,K,D*20
$GNVTG,348.78,T,,M,0.07,N,0.13,K,D*23
$GNVTG,348.78,T,,M,0.10,N,0.19,K,D*2F
$GNVTG,348.78,T,,M,0.05,N,0.09,K,D*2A
$GNVTG,348.78,T,,M,0.10,N,0.18,K,D*2E

The D in the 9th field is the FAA mode indicator, with the checksum moved to field 10.

The same problem probably exists for the BWC, BWR, GLL, RMA, RMB, WCV, and XTE sentences. For now, VTG is the only one I've had a chance to fix and test.