Bluetooth: 6lowpan: Remove unnecessary peer lookup

During chan_recv_cb there is already a peer lookup which can be passed
to recv_pkt directly instead of the channel.

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>