net/mlx5: fix crash during RETA update

net/mlx5: fix crash during RETA update

Reta update needs to stop/start the port but stopping the port does not
disable the polling functions which may end in a segfault if a core is
polling the queue while the control thread is modifying it.

This patch changes the sequences to an order where such situation cannot
happen.

Fixes: aa13338faf5e ("net/mlx5: rebuild flows on updating RETA")
Cc: stable@dpdk.org
Signed-off-by: Nelio Laranjeiro <nelio.laranjeiro@6wind.com>
Acked-by: Yongseok Koh <yskoh@mellanox.com>