Problem to change flight modes

#### Issue details
_Please describe the problem, or desired feature_

Currently, I see changing channel 5 value in Mission Planner, in particular in the Radio Calibration menu, but the flight modes in Flight Modes menu don't change and the Current PWM is always 0. I don't know if there is a relation whit this problem and the error PreArm that I have permanently.

#### Version
_What version was the issue encountered with_

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

#### Airframe type
_What type of airframe (flying wing, glider, hex, Y6, octa etc)_
hex
#### Hardware type
_What autopilot hardware was used? (pixhawk, pixracer, PX4FMU etc)_
APM 2.8
#### Logs
_Please provide a link to any relevant logs that show the issue_

Start by reproducing the issue in Mission Planner's Radio Calibration and Flight Modes menus with a Copter hex airframe, APM 2.8 hardware, and channel 5 input. Compare the channel 5 value, Current PWM, flight-mode selection, and persistent PreArm error; done means the cause is identified and the affected values or error behavior are corrected.

Written by the indexing model from the issue text.