DolphinWX: Fix a mix-up

Caused by a commit of mine, cb3b1b6

Source/Core/DolphinWX/GameListCtrl.cpp

@@ -956,7 +956,7 @@ void CGameListCtrl::OnRightClick(wxMouseEvent& event)
         popupMenu.AppendCheckItem(IDM_SET_DEFAULT_ISO, _("Set as &default ISO"));
 
       // First we have to decide a starting value when we append it
-      if (platform == SConfig::GetInstance().m_strDefaultISO)
+      if (selected_iso->GetFileName() == SConfig::GetInstance().m_strDefaultISO)
         popupMenu.FindItem(IDM_SET_DEFAULT_ISO)->Check();
 
       popupMenu.AppendSeparator();