ieee802154: add basic support for RF212 to at86rf230 driver

Since the AT86RF2xy chips are mostly compatible, this is only a small
change to the actual driver code. The at86rf230 driver already supports the
RF212 in most places, only three small adjustments are required:

 * force the initial state after P_ON to FORCE_TRX_OFF to work around a
   documented erratum
 * channels_supported depends on the frequency of the transceiver, and
   thus is_rf212
 * do early detection of chip version select an appropriate _ops struct
   based on the chip version.
Signed-off-by: Phoebe Buckheister <phoebe.buckheister@itwm.fraunhofer.de>
Signed-off-by: David S. Miller <davem@davemloft.net>