Receiver to GNURadio via Ethernet @48KSPS

* Actually turn on the ADC while receiving.
* Add Push-to-Listen (PTL) to whiteboxd.
* Ability to turn on/off RX calibration for CMX991.
* Only update CMX991 registers that have changed.
* Update poll() to show readable/writeable when idle.
* No more auto-engagement of PTT.