net/mlx5: preserve promiscuous flag for flow isolation mode

net/mlx5: preserve promiscuous flag for flow isolation mode

mlx5_dev_ops_isolate doesn't have APIs for enabling/disabling promiscuous
mode as it can't be enabled in flow isolation mode. If the function
pointers are null, librte APIs such as rte_eth_promiscuous_enable/disable()
fail to set the flag (dev->data->promiscuous). The flag is used when
starting traffic by mlx5_traffic_enable(). When switching out of flow
isolation mode, promiscuous mode will not be set even though it has been
enabled.

Fixes: 0887aa7f27f3 ("net/mlx5: add new operations for isolated mode")
Cc: stable@dpdk.org
Signed-off-by: Yongseok Koh <yskoh@mellanox.com>