Last fix to eliminate a warning also perpetrated a logic error in semaphore handling.

wireless/ieee802154/mac802154_netdev.c

@@ -443,9 +443,10 @@ static int macnet_notify(FAR struct mac802154_maccb_s *maccb,
 #endif
         }
 #endif
+
+      nxsem_post(&priv->md_exclsem);
     }
 
-  nxsem_post(&priv->md_exclsem);
   return OK;
 }