6lowpan: Only make 6lowpan links to IEEE802154 devices

Refuse to create 6lowpan links if the actual hardware interface is
of any type other than ARPHRD_IEEE802154.

Signed-off-by: Alan Ott <alan@signal11.us>
Suggested-by: Alexander Aring <alex.aring@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>