ieee802154: 6lowpan: fix non-lowpan wpan interfaces

We receive all 802.15.4 frames on the packet handler "lowpan_rcv" this
patch checks if the wpan device belongs to a lowpan interface.

Signed-off-by: Alexander Aring <alex.aring@gmail.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>