net/mlx4_en: Don't use vlan tag value as an indication for vlan presence

The vlan tag can be zero. This is why it can't serve as an indication
that packet requires VLAN header in the TX flow.

Signed-off-by: Moni Shoua <monis@mellanox.com>
Signed-off-by: Or Gerlitz <ogerlitz@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>