net/mlx4_en: Notify TX Vlan offload change

Notify users when TX vlan offload feature changed with ethtool.
Relevant command - ethtool -K <eth> txvlan on/off.

Signed-off-by: Ido Shamay <idos@mellanox.com>
Signed-off-by: Amir Vadai <amirv@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>