Bluetooth: Fix notifying discovery state when powering off

The discovery state should be set to stopped when the HCI device is
powered off. This patch adds the appropriate call to the
hci_discovery_set_state() function from hci_dev_do_close() which is
responsible for the power-off procedure.

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