RC_Channel: re-instate init_aux_function call.

[IamPete1]

This fixs a bug added by me in #28655. The init aux function call was replaced by run aux function. Init mostly just calls run, however for invalid options it does not. This fixes the "Invalid channel option (0)" warning.

In the longer term I would like to consolidate the init calls such that each function can decide based on the trigger source.

[peterbarker]

Board                    AP_Periph  blimp  bootloader  copter  heli  iofirmware  plane  rover  sub
CubeOrange-periph-heavy  *                                                                     
CubeRedPrimary                      -8     *           0       0                 0      -8     -8
Durandal                            -56    *           -56     -56               -56    -56    32
Hitec-Airspeed           *                                                                     
KakuteH7-bdshot                     0      *           0       0                 -8     0      -8
MatekF405                           -8     *           0       0                 0      -8     -8
Pixhawk1-1M-bdshot                  0                  -8      0                 0      0      -8
f103-QiotekPeriph        *                                                                     
f303-Universal           *                                                                     
iomcu                                                                *                         
revo-mini                           -8     *           0       0                 0      0      0
skyviper-journey                                       -8                                      
skyviper-v2450                                         -8                                      

[rishabsingh3003]

I just noticed this on a new build and was wondering where it came from..