net/mlx4: fix empty Ethernet spec with VLAN

net/mlx4: fix empty Ethernet spec with VLAN

When the ETH spec is empty MLX4 PMD doesn't allow match other criteria,
which means the flow should be promisc one.

Currently, PMD validates this by setting flow->promisc bit when ETH spec
is empty and checking whether there is other rte_flow_item followed
when flow->promisc is on.

However, commit [1] adds support to match traffic only on VLAN id, the
above validation logic should be changed accordingly.

This patch changes the above validate logic by skipping flow->promisc
check if this item is VLAN.

[1]:
Fixes: c0d239263156 ("net/mlx4: support flow w/o ETH spec and with VLAN")
Cc: stable@dpdk.org
Signed-off-by: Xiaoyu Min <jackmin@nvidia.com>
Acked-by: Matan Azrad <matan@nvidia.com>