Fix state restoration in EngineControl dialog for backends with driver selection

We want to save the driver name even when the engine is running and the driver
combo is insensitive

gtk2_ardour/engine_dialog.cc

@@ -2405,7 +2405,7 @@ EngineControl::get_backend () const
 string
 EngineControl::get_driver () const
 {
-	if (driver_combo.get_sensitive() && driver_combo.get_parent()) {
+	if (driver_combo.get_parent()) {
 		return driver_combo.get_active_text ();
 	} else {
 		return "";