Avoid double call to set ofono's online property

On initialization a double call to set-soft was being done for the ofono device. This caused bugs LP #1392397 and #1379807.
The callback is not going to be cancelled anymore. Instead, g_object_ref() is called before doing the proxy call so the object is known to exist. So we can now access user_data. If not, it is an error in the logic.