MP setting RC9_TRIM ~ RC12_TRIM = 0

While investigating [a report that perhaps MP's build-in simulator wasn't working while using a joystick](https://discuss.ardupilot.org/t/copter-4-0-1-rc3-available-for-beta-testing/51295/3) I found that I was getting a pre-arm error, "RC12_TRIM lower than RC12_MIN".

The issue appears to be caused by MP's (I'm using 1.3.70 build 1.3.7329.40911) RC calibration feature. To recreate the problem do this:

- Start MP's Simulator and ensure the "wipe" checkbox is checked
- Use Config, Full Parameter Tree and check RC9_TRIM (should be 1500) and RC9_MIN (should be 1000)
- Setup a joystick so that RC input channels 1 to 4 are assigned to sticks on the joystick
- Use MP's Hardware Config, Mandatory Hardware, Radio Calibration page to calibrate the RC input
- check RC9_TRIM and RC9_MIN again and note that RC9_TRIM has been changed to 0

P.S. I thought we'd already dealt with this issue (https://github.com/ArduPilot/MissionPlanner/issues/1853) but I guess it was either never fixed for it's some slight variation on the earlier problem.

Start by reproducing the issue in Mission Planner's Simulator with the wipe option enabled, then trace the Radio Calibration flow reached through Hardware Config > Mandatory Hardware. Verify that calibrating joystick inputs on channels 1–4 does not set RC9_TRIM through RC12_TRIM to 0 or below the corresponding minimum values, and confirm the pre-arm error is gone.

Written by the indexing model from the issue text.