Bluetooth: mgmt: Add support for Set Link Security command

The Set Link Security mgmt command is used to enable or disable link
level security, also known as Security Mode 3. This is rarely enabled in
modern systems but the command needs to be available for completeness,
qualification purposes and those few systems that actually want to
enable it.

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