drivers/at86rf231: refactoring of the at86rf231 radio driver

* deploy extended operation mode
* cleanup
* implement netdev 802154.h interface
Where does `netdev_802154_driver_t` come from? `git grep netdev_802154_driver_t` did not show me the definition.

Anyway, I'd prefer designated initializers as in '.init = at86rf231_initialize,` `.send = netdev_802154_send_data`.