wifi: (reverts !899) Avoid return value conflicting with Python bindings

As discussed in #613, MR !899 introduced a return value that is causing
problems with the Python bindings.

src/wifi/model/wifi-mode.cc

@@ -136,7 +136,7 @@ WifiMode::GetConstellationSize (void) const
   return item->GetConstellationSizeCallback ();
 }
 
-const std::string&
+std::string
 WifiMode::GetUniqueName (void) const
 {
   //needed for ostream printing of the invalid mode

src/wifi/model/wifi-mode.h

@@ -127,7 +127,7 @@ public:
    * \returns a human-readable representation of this WifiMode
    * instance.
    */
-  const std::string& GetUniqueName (void) const;
+  std::string GetUniqueName (void) const;
   /**
    * \returns true if this mode is a mandatory mode, false
    *          otherwise.

src/wifi/model/wifi-tx-vector.cc

@@ -337,7 +337,7 @@ WifiTxVector::IsValid (void) const
     {
       return false;
     }
-  const std::string& modeName = m_mode.GetUniqueName ();
+  std::string modeName = m_mode.GetUniqueName ();
   if (m_channelWidth == 20)
     {
       if (m_nss != 3 && m_nss != 6)