Camera Gimbal fail to load higher RC Channels.

To reproduce: (in this case Fresh installed arducopter.)
-connect, open initial setup, optional setup, camera gimbal
-assign tilt to ch 10 , roll=ch11 , pan=ch12 trigger=ch13
-restart AP2 (otherwise it will just keep settings)
-connect, and go to "camera gimbal"
-observe that roll appears as assigned to RC11 , but nothing else.

full parameter list confirms that the RCx_FUNCTION parameters have been correctly set, but are not displayed.

Reproduce the issue in a fresh ArduCopter setup by assigning tilt, roll, pan, and trigger to RC channels 10–13 in Initial Setup > Optional Setup > Camera Gimbal, then restart APM Planner and reconnect. Compare the Camera Gimbal display with the full parameter list, especially the RCx_FUNCTION values; done means all assigned channels are displayed correctly.

Written by the indexing model from the issue text.