SRV_Channel: pwm_from_angle: return trim for 0 high_out

[IamPete1]

When changing output function from disabled to scaled output high_out is zero until we call update_aux_servo_function which is done a a lower rate than the outputs. So we end up with a divide by 0 for a few loops.

pwm_from_range already checks for high_out of 0.

#21347 (although I suspect setting a function to scaled output is not the only way to hit this.)

Found when checking we can do full range servo for #22011