wifi: (fixes #880) Init last busy structs at initialization time

Otherwise, channel changes done after Install() and before Initialize()
do not trigger an update of the last busy structs.

src/wifi/model/channel-access-manager.cc

@@ -139,6 +139,13 @@ ChannelAccessManager::~ChannelAccessManager()
     m_phyListener = nullptr;
 }
 
+void
+ChannelAccessManager::DoInitialize()
+{
+    NS_LOG_FUNCTION(this);
+    InitLastBusyStructs();
+}
+
 void
 ChannelAccessManager::DoDispose()
 {

src/wifi/model/channel-access-manager.h

@@ -261,6 +261,7 @@ class ChannelAccessManager : public Object
     bool IsBusy() const;
 
   protected:
+    void DoInitialize() override;
     void DoDispose() override;
 
   private:

src/wifi/model/wifi-mac.cc

@@ -363,6 +363,14 @@ WifiMac::DoInitialize()
     {
         it->second->Initialize();
     }
+
+    for (auto& link : m_links)
+    {
+        if (auto cam = link->channelAccessManager)
+        {
+            cam->Initialize();
+        }
+    }
 }
 
 void