AP_Camera: RunCam: get rpty channels directly using convenience functions

[peterbarker]

Extracted from #28089

[peterbarker]

Board                    AP_Periph  blimp  bootloader  copter  heli  iofirmware  plane  rover  sub
CubeOrange-periph-heavy  *
Durandal                            -96    *           -104    -104              -96    -104   -104
Hitec-Airspeed           *
KakuteH7-bdshot                     -64    *           -64     -64               -64    -56    -64
MatekF405                           -56    *           -64     -64               -64    -64    -56
Pixhawk1-1M-bdshot                  *                  *       *                 *      *      *
f103-QiotekPeriph        *
f303-Universal           *
iomcu                                                                *
revo-mini                           -56    *           -56     -64               -64    -64    -64
skyviper-v2450                                         *

[peterbarker]

@andyp1per this should be NFC, but I don't have the hardware and we don't have a sim.

Simply navigating the menus should be enough to test, and I think you have a working setup?