Bluetooth: 6lowpan: Do not free skb when packet is dropped

If we need to drop the message because of some error in the
compression etc, then do not free the skb as that is done
automatically in other part of networking stack.

Signed-off-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>