net/tcp: reset the connection ref count before tcp_free()

reset the connection refcount if SYN retry count has elapsed

Assertion:

up_assert: Assertion failed at file:tcp/tcp_conn.c line: 764 task: netdev_wq

N/A

Change-Id: I3b0966af52a9dc52d459e8b5aa7d478955d6691f
Signed-off-by: chao.an <anchao@xiaomi.com>

net/tcp/tcp_timer.c

@@ -270,6 +270,7 @@ void tcp_timer(FAR struct net_driver_s *dev, FAR struct tcp_conn_s *conn,
 
                   /* Finally, we must free this TCP connection structure */
 
+                  conn->crefs = 0;
                   tcp_free(conn);
                   goto done;
                 }