Bluetooth: Update class of device on discoverable timeout

When the discoverable timeout triggers and limited discoverable mode
was used, then the class of device needs to be updated to remove
the limited discoverable bit.

To keep the class of device logic in a central place, expose a new
function mgmt_discoverable_timeout that can be called from the
timeout callback. In case the class of device value needs updating,
it will add the HCI command to the transaction.

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