[Bluetooth] stopDiscovery API fix to be compliant with TCT tests

stopDiscovery API is supposed to cancel an active discovery session.
If user cancels the discovery while there is no active discovery,
we should NOT return an error to pass TCT tests.
Merge pull request #349 from eurogiciel-oss/stopDiscovery_noerror

[Bluetooth] stopDiscovery API fix to be compliant with TCT tests