ieee802154: interface type to be added

This stack implementation distinguishes several types of slave
interfaces. Another parameter to 'add_iface_' function is added
to clarify the interface type is going to be registered.

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