no Flight Modes when using ArduSub

#### Issue details
there are not flight modes (it's empty) to choose from.
1.Action -> set mode
2.Configuration ->Mandatory -> Flight Modes
3.Unknown in HUD

#### Version
1.3.49.1

#### Platform
[ ] All
[ ] AntennaTracker
[ ] Copter
[ ] Plane
[ ] Rover
[ X] Sub

#### Airframe type
_What type of airframe (flying wing, glider, hex, Y6, octa etc)_

#### Hardware type
pixhawk v2

#### Logs
_Please provide a link to any relevant logs that show the issue_

Start by reproducing the issue in Mission Planner 1.3.49.1 with a Pixhawk v2 and an ArduSub vehicle. Check Action → Set mode, Configuration → Mandatory → Flight Modes, and the HUD, recording the empty mode list and Unknown state. Done means identifying the cause and verifying that the available modes and HUD state are correct.

Written by the indexing model from the issue text.