Bluetooth: Fix RFCOMM usage of in-kernel L2CAP sockets

The CID value of L2CAP sockets need to be set to zero. All userspace
applications do this via memset() on the sockaddr_l2 structure. The
RFCOMM implementation uses in-kernel L2CAP sockets and so it has to
make sure that l2_cid is set to zero.

Signed-off-by: Marcel Holtmann <marcel@holtmann.org>