Bluetooth: Use {start,stop}_discovery_complete handler for cmd_complete

Sending the required cmd_complete for the management commands should be
done in one place and not in multiple places. Especially for Start and
Stop Discovery commands this is split into to sending it in case of
failure from the complete handler, but in case of success from the
event state update function triggering mgmt_discovering. This is way
too convoluted and since hci_request serializes the HCI command
processing, send the cmd_complete response from the complete handler
for all cases.

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