net/mlx5: allow push VLAN without VID

net/mlx5: allow push VLAN without VID

Currently the push VLAN action requires a VID value, either from
existing VLAN item, or from following 'set VLAN vid' action.

This patch removes this limitation, allowing a push VLAN action with
vid value 0.

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