Plane::send_nav_controller_output uses incorrect airspeed units

#### Issue details
Plane::send_nav_controller_output reports airspeed error in [cm] whereas mavlink specifies it in [m].
I did a (very brief) search in Mission Planner, qGroundControl and Droid Planner/Tower to check that they do not use this message so I believe there should be no problems with correcting it.

#### Version
3.8.4

#### Platform
[ ] All
[ ] AntennaTracker
[ ] Copter
[x] Plane
[ ] Rover
[ ] Submarine

Locate Plane::send_nav_controller_output and inspect how its airspeed error is reported against the MAVLink message specification. Confirm the expected unit is meters and verify that the resulting message uses that unit; the issue does not name a test file or other entry point.

Written by the indexing model from the issue text.