Bluetooth: 6lowpan: change netdev_priv to lowpan_dev

The usually way to get the btle lowpan private data is to use the
introduced lowpan_dev inline function. This patch will cleanup by using
lowpan_dev consequently.

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