mac802154: add wpan device-class support

Every real 802.15.4 transceiver, which works with software MAC layer,
can be classified as a wpan device in this stack. So the wpan device
implementation provides missing link in datapath between the device
drivers and the Linux network queue.

According to the IEEE 802.15.4 standard each packet can be one of the
following types:
 - beacon
 - MAC layer command
 - ACK
 - data

This patch adds support for the data packet-type only, but this is
enough to perform data transmission and receiving over radio.

Signed-off-by: Alexander Smirnov <alex.bluesman.smirnov@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>