RC input reverse checkboxes have hardcoded channels

#### Issue details
I have custom RCMAP (TAER) and when I reverse channel on radio calibration tab it's going to different channel because RCMAP is not used.

#### Version
latest 1.3.75 build 1.3.7883.26333

#### Platform
[ X] All (most probalbly)
[ ] AntennaTracker
[ ] Copter
[ X] Plane (checked on plane)
[ ] Rover
[ ] Sub

#### Airframe type
Airplane, wing

#### Hardware type
pixracer

Start in Mission Planner's Radio Calibration tab and reproduce the issue with a custom RCMAP (TAER) on a Plane airframe by reversing a channel. Done means the reversed input targets the channel defined by RCMAP rather than a hardcoded channel.

Written by the indexing model from the issue text.