ieee802154: 6lowpan: remove multiple lowpan per wpan support

We currently supports multiple lowpan interfaces per wpan interface. I
never saw any use case into such functionality. We drop this feature now
because it's much easier do deal with address changes inside the under
laying wpan interface.

This patch removes the multiple lowpan interface and adds a lowpan_dev
netdev pointer into the wpan_dev, if this pointer isn't null the wpan
interface belongs to the assigned lowpan interface.

Reviewed-by: Stefan Schmidt <stefan@osg.samsung.com>
Tested-by: Stefan Schmidt <stefan@osg.samsung.com>
Signed-off-by: Alexander Aring <alex.aring@gmail.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>