Bluetooth: Default to no security with L2CAP RAW sockets

L2CAP RAW sockets can be used for things which do not involve
establishing actual connection oriented L2CAP channels. One example of
such usage is the l2ping tool. The default security level for L2CAP
sockets is LOW, which implies that for SSP based connection
authentication is still requested (although with no MITM requirement),
which is not what we want (or need) for things like l2ping. Therefore,
default to one lower level, i.e. BT_SECURITY_SDP, for L2CAP RAW sockets
in order not to trigger unwanted authentication requests.

Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>