change $this->$module->config() to check settings (bugfix) in libraries/fuel_advanced_module

I found that with my new advanced module (invoices), settings were not being taken into account when calling $this->invoices->config(), instead just the config was used. I traced it to theis fix.  Changing _settings to settings() because _ settings is NULL until settings() grabs the settings.
Hope this is useful!
Thx Guy