Bluetooth: Add chan->ops->set_shutdown()

We need to remove all direct access of struct sock from L2CAP core.
This change is pretty simple and just add a new L2CAP channel callback to
do the work in the L2CAP socket side.

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