net/mlx5: preserve allmulticast flag for flow isolation mode

net/mlx5: preserve allmulticast flag for flow isolation mode

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