net/mlx4_en: Enable DCB ETS ops only when supported by the firmware

Enable the DCB ETS ops only when supported by the firmware. For older firmware/cards
which don't support ETS, advertize only PFC DCB ops.

Signed-off-by: Eugenia Emantayev <eugenia@mellanox.co.il>
Signed-off-by: Or Gerlitz <ogerlitz@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>