Bluetooth: Handle security level 4 for L2CAP connections

With the introduction of security level 4, the L2CAP sockets need to
be made aware of this new level. This change ensures that the pairing
requirements are set correctly for these connections.

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