ieee802154: at86rf230: add support for rf233 chip

The rf233 and rf231 are sufficiently similar that we can treat
rf233 like rf231.

rf233 is missing some features that rf231 has, but we don't currently
make use of them so there's nothing to handle differently yet.

Should we add support in the future for rf231 *_NOCLK or SLEEP states,
or PAD_IO drive strength, exceptions will need to be made for rf233.

Signed-off-by: Thomas Stilwell <stilwellt@openlabs.co>
Signed-off-by: David S. Miller <davem@davemloft.net>