Bluetooth: Fix setting STOPPING state for discovery

If any of the HCI commands from the hci_stop_discovery function were
successfully sent we need to set the discovery state to STOPPING. The
Stop Discovery code was already handling this, but the code in
clean_up_hci_state was not. This patch updates the hci_stop_discovery to
return a bool to indicate whether it queued any commands and the
clean_up_hci_state() function respectively to look at the return value
and call hci_discovery_set_state() if necessary.

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