AR_Motors: fix brushed motor support for omni vehicles

[mattp256]

Fixes an issue where omni motors (BrushedWithRelay/BrushedBiplor) are not configured correctly.

I ran into the same problem another user mentioned here: OmniX frame using MOT_PWM_TYPE = 3 (BrushedWithRelay) only outputs RC servo pulses instead of a duty cycle PWM.

Proposed fix worked great when tested with Cytron MDD10A motor driver.

[IamPete1]

This looks like a init ordering issue to me. We setup the frame type here:

ardupilot/libraries/AR_Motors/AP_MotorsUGV.cpp

Line 146 in a829cd3

setup_omni();

But we do the PWM setup a few lines above here:

ardupilot/libraries/AR_Motors/AP_MotorsUGV.cpp

Line 139 in a829cd3

setup_pwm_type();

So _motors_num is not set when its used. Can you test if moving setup_omni to the top of the function fixes it too.

[mattp256]

Yes exactly, _motors_num is not yet set at the time it is used in setup_pwm_type. Moving setup_omni ahead of setup_pwm_type is also confirmed to fix the problem. I was worried there may be some other unintended side effect if I changed the init order so thought forcing the for loop to iterate may be the least disruptive fix - I suppose that carries its own risk of side effects though. Let me know if you prefer changing the init order instead and I will update the PR.