Bluetooth: 6lowpan: Use netif APIs to flow control

Rely on netif_wake_queue and netif_stop_queue to flow control when
transmit resources are unavailable.

Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>
Acked-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>