net/mlx5: fix push VLAN action to use item info

net/mlx5: fix push VLAN action to use item info

Currently when PMD create push VLAN action it need to provide VID to HW
and PMD get VID value from item VLAN in pattern if there is no
of_set_vlan_vid action following.

When user create rule like [1], which has of_set_vlan_vid action
before of_push_vlan, the intention is to modify VID on existing VLAN
header and push a new VLAN header with VID _inherit_ from the previous
of_set_vlan_vid.

Currently the above is not covered by PMD, PMD always fetch the VLAN
information from item for of_push_vlan action.

Fix it by only fetch VLAN information from item when there is no
previous of_set_vlan_vid action.

[1]: testpmd> flow create 2 ingress transfer group 1 priority 3 pattern
                eth / vlan vid is 2731 / ipv4 / end actions
		of_set_vlan_vid vlan_vid 3209 / of_push_vlan ethertype
		0x88A8 / port_id id 1 / end

Fixes: b8c0372bc5ac ("net/mlx5: fix set VLAN ID/PCP in new header")
Cc: stable@dpdk.org
Signed-off-by: Xiaoyu Min <jackmin@mellanox.com>
Reviewed-by: Dekel Peled <dekelp@mellanox.com>
Acked-by: Viacheslav Ovsiienko <viacheslavo@mellanox.com>