Bluetooth: Do not access chan->sk directly

In the process of removing socket usage from L2CAP we now access the L2CAP
socket from the data member of struct l2cap_chan. For the L2CAP socket
user the data member points to the L2CAP socket.

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