6lowpan: move lowpan_802154_dev to 6lowpan

This patch moves the 802.15.4 link layer specific structures to generic
6lowpan. This is necessary for special 802.15.4 6lowpan handling in
6lowpan generic layer.

Reviewed-by: Stefan Schmidt <stefan@osg.samsung.com>
Signed-off-by: Alexander Aring <aar@pengutronix.de>
Acked-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>