Make mqtt websocket subscribe use callback functions (#87885)

Nothing was being awaited so this was needlessly generating tasks when the callbacks fired

homeassistant/components/mqtt/__init__.py

@@ -596,7 +596,8 @@ async def websocket_subscribe(
     if not connection.user.is_admin:
         raise Unauthorized
 
-    async def forward_messages(mqttmsg: ReceiveMessage) -> None:
+    @callback
+    def forward_messages(mqttmsg: ReceiveMessage) -> None:
         """Forward events to websocket."""
         try:
             payload = cast(bytes, mqttmsg.payload).decode(