Bluetooth: 6lowpan: Set tx_queue_len to DEFAULT_TX_QUEUE_LEN

Make netdev queue packets if we run out of credits.

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>