Bluetooth: Reuse hci_stop_discovery function when cleaning up HCI state

When cleaning up the HCI state as part of the power-off procedure we can
reuse the hci_stop_discovery() function instead of explicitly sending
HCI command related to discovery. The added benefit of this is that it
takes care of canceling name resolving and inquiry which were not
previously covered by the code.

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