Bluetooth: 6lowpan: always check destination address

BLE based 6LoWPAN networks are highly constrained in bandwidth.
Do not take a short-cut, always check if the destination address is
known to belong to a peer.

As a side-effect this also removes any behavioral differences between
one, and two or more connected peers.

Acked-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>
Tested-by: Michael Scott <mike@foundries.io>
Signed-off-by: Josua Mayer <josua.mayer@jm0.eu>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>