wifi: Move m_stateLogger to private in WifiPhyStateHelper

src/wifi/model/wifi-phy-state-helper.h

@@ -186,9 +186,6 @@ public:
    */
   void SwitchFromSleep (Time duration);
 
-  /** \todo Why is this public? */
-  TracedCallback<Time, Time, WifiPhy::State> m_stateLogger;
-
   /**
    * TracedCallback signature for state changes.
    *
@@ -291,6 +288,11 @@ private:
    */
   void DoSwitchFromRx (void);
 
+  /**
+   * The trace source fired when state is changed.
+   */
+  TracedCallback<Time, Time, WifiPhy::State> m_stateLogger;
+
   bool m_rxing; ///< receiving
   bool m_sleeping; ///< sleeping
   Time m_endTx; ///< end transmit