Revist:Back out fix for secondary issue

 Ultimate we want this changes that is being backed out herein.
 But it is breaking things because it returns the EINVAL
 when there is a rate overlap. So the rest of the pwm ioctl
 calls then fail and do not set the pulse widths on arming.

 As a secondary issue. We sould call up_pwm_servo_init() to
 establish the PWM channel allocation early. This then allows
 FMU::set_pwm_rate to properly check for improper rate request
 not isolate to one group (timer).