Fix "could not extract FPS"

At least with libroyale 3.4.0, the previous code left the variable
`name` always empty.

Author: mintar

src/pico_flexx_driver.cpp

@@ -747,8 +747,7 @@ class PicoFlexx : public royale::IDepthDataListener, public royale::IExposureLis
     }
     OUT_INFO("use case changed to: " FG_YELLOW << useCases[idx]);
 
-    std::string name;
-    useCases[idx].toStdString(name);
+    std::string name = royale::String::toStdString(useCases[idx]);
     size_t end = name.find("FPS");
     size_t start = name.rfind('_', end);