Bluetooth: 6lowpan: Fix possible race

This patch fix a possible race after calling register_netdev. After
calling netdev_register it could be possible that netdev_ops callbacks
use the uninitialized private data of lowpan_dev. By moving the
initialization of this data before netdev_register we can be sure that
initialized private data is be used after netdev_register.

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