Bluetooth: Update class of device after changing discoverable mode

When the discoverable mode gets changed, ensure that the class of
device value has the correct limited discoverable bit value set.

Since the class of device HCI command will only be send to the
controller when the value changes, it is safe to just always
trigger the update.

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