Auto assign wii controllers in netplay

Just copy the gamecube logic and automatically assign players to the first empty slot.

More consistent with GameCube and helps Android hosts where there is no UI for managing controller mappings currently.

Source/Core/Core/NetPlayServer.cpp

@@ -2272,6 +2272,15 @@ void NetPlayServer::AssignNewUserAPad(const Client& player)
       break;
     }
   }
+  for (PlayerId& mapping : m_wiimote_map)
+  {
+    // 0 means unmapped
+    if (mapping == 0)
+    {
+      mapping = player.pid;
+      break;
+    }
+  }
 }
 
 PlayerId NetPlayServer::GiveFirstAvailableIDTo(ENetPeer* player)