Encode RC type in RSSI field for GCS
Force RSSI to zero if RC is lost
Initialize RSSI so it doesn't remain uninitialized

Spektrum doesn’t support rssi so it is not set by st24_decode.
Merge pull request #1431 from DonLakeFlyer/rctype

Initialize RSSI so it doesn't remain uninitialized
Correct setting of DSM and ST24 flags
Merge pull request #1460 from DonLakeFlyer/rctype

Correct setting of DSM and ST24 flags
Merge pull request #1420 from PX4/rctype

Encode RC type in RSSI field for GCS