Rover: brushed bipolar motor is outputting SERVOx_MIN at startup instead of SERVOx_TRIM

[rmackay9]

This was raised as part of Rover-4.1.0 beta testing here: https://discuss.ardupilot.org/t/brushedbipolar-pwm-problem/76153

[IamPete1]

I cant reproduce on master, I don't see any path for the trim param to be changed. I could believe that were outputting normal PWM for a bit before switching to BRUSHED_BIPOLAR.

[IamPete1]

Its falling fowl of this trim set.

ardupilot/libraries/SRV_Channel/SRV_Channel_aux.cpp

Line 255 in d853d16

c.servo_trim.set(1000);

Only called by FETtec and BLHeli drivers, which is why there was no reproduction in SITL.