Radio Calibration Bug & Improvment

#### Issue details
Desired Feature

Radio Calibration is mandatory thing to set up airframes
and most of users does use equal or less than 8 Channel RC receivers.
However, current MP does update every 16 input channels available in ArduPilot
regardless of channel input is updated or not,
so consequently making some annoying error in
PreArm RC Check for RC channel 9 to 16 trim pwm (usually 974) is less than min pwm.

I think this should be updated based on RC channel available maybe checking certain channel pwm input is out of range or whether channels updated

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

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

#### Airframe type
_What type of airframe (flying wing, glider, hex, Y6, octa etc)_
Hexa
#### Hardware type
_What autopilot hardware was used? (pixhawk, pixracer, PX4FMU etc)_

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

Start by locating Mission Planner's radio calibration implementation and tracing how it processes all 16 ArduPilot input channels. Reproduce the issue with a receiver using eight or fewer channels, then verify that unavailable channels no longer trigger RC trim PWM pre-arm errors while available-channel calibration still works.

Written by the indexing model from the issue text.