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.
Right. In fact I already tried the custom struct thing, but it is a bit ugly as you need to have cross references between that custom struct and the UrfDeviceOfono struct to track object state.

As I said in a previous comment, I will try with calls to g_object_ref()/g_object_unref() in the caller and in the callback, seems to me a more elegant solution.