net/mlx5: fix bit mask to validate push VLAN

net/mlx5: fix bit mask to validate push VLAN

Validation function of 'push VLAN' action uses (POP_VLAN or PUSH_VLAN)
actions flags, instead of using a mask of both flags.

This patch replaces it to use existing VLAN_ACTIONS mask.

Fixes: 9aee7a8418d4 ("net/mlx5: support push flow action on VLAN 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>