Bluetooth: Fix memory leak with L2CAP signal channels

The wrong type of L2CAP signalling packets on the wrong type of
either BR/EDR or LE links need to be dropped. When that happens
the packet is dropped, but the memory not freed. So actually
free the memory as well.

Signed-off-by: Marcel Holtmann <marcel@holtmann.org>
Signed-off-by: Gustavo Padovan <gustavo.padovan@collabora.co.uk>