Replace std::ostringstream usage with fmt::format

Source/Core/InputCommon/ControllerInterface/CoreDevice.cpp

@@ -162,15 +162,12 @@ void Device::AddCombinedInput(std::string name, const std::pair<std::string, std
 std::string DeviceQualifier::ToString() const
 {
   if (source.empty() && (cid < 0) && name.empty())
-    return "";
+    return {};
 
-  std::ostringstream ss;
-  ss << source << '/';
   if (cid > -1)
-    ss << cid;
-  ss << '/' << name;
-
-  return ss.str();
+    return fmt::format("{}/{}/{}", source, cid, name);
+  else
+    return fmt::format("{}//{}", source, name);
 }
 
 //