Prior to onshot being added to the system. The -r of the
   "rate" command would not invoke the ioctl PWM_SERVO_SET_UPDATE_RATE
   when -r was not provided on the command line. This may have been a
   feature or a bug.

   When onshot was added to the pwm command a bug was intorduced
   that precluded the ioctl PWM_SERVO_SET_UPDATE_RATE from being
   called on -r 0.

   This commit fixes that issue, and preserves the "prio to oneshot"
   behavior of the "rate" command when -r is not specified.