Bluetooth: Move Stop Discovery to req_workqueue

Since discovery also deals with LE scanning it makes sense to move it
behind the same req_workqueue as other LE scanning changes. This also
simplifies the logic since we do many of the actions in a synchronous
manner.

Part of this refactoring is moving hci_req_stop_discovery() to
hci_request.c. At the same time the function receives support for
properly handling the STOPPING state since that's the state we'll be
in when stopping through the req_workqueue.

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