6lowpan: Don't set IFF_NO_QUEUE

There is no point in setting IFF_NO_QUEUE should already have taken
care of setting it if tx_queue_len is not set, in fact this may
actually disable queue for interfaces that require it and do set
tx_queue_len.

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>