net: use netdev_lock to protect netdev_txnotify_dev

since the type of sconn lock is rmutex, there is no need to release
sconn lock before TX

Signed-off-by: zhanghongyu <zhanghongyu@xiaomi.com>

net/tcp/tcp_timer.c

@@ -154,7 +154,9 @@ static void tcp_timer_expiry(FAR void *arg)
         {
           tcp_conn_list_unlock();
           conn->timeout = true;
+          netdev_lock(conn->dev);
           netdev_txnotify_dev(conn->dev);
+          netdev_unlock(conn->dev);
           return;
         }
     }