net/mlx4: fix default flow rule create

Original patch changed logic of function mlx4_flow_merge_eth().
The setting of flow->promisc was wrongly removed.
This patch adds the removed setting of flow->promisc, to restore
the required behavior.

Fixes: c0d239263156 ("net/mlx4: support flow w/o ETH spec and with VLAN")
Cc: stable@dpdk.org
Signed-off-by: Dekel Peled <dekelp@mellanox.com>
Acked-by: Yongseok Koh <yskoh@mellanox.com>