net/mlx4_en: Add DCB PFC support through CEE netlink commands

This patch adds support for reading and updating priority flow
control (PFC) attributes in the driver via netlink.

Signed-off-by: Rana Shahout <ranas@mellanox.com>
Signed-off-by: Eran Ben Elisha <eranbe@mellanox.com>
Signed-off-by: Eugenia Emantayev <eugenia@mellanox.com>
Signed-off-by: Tariq Toukan <tariqt@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>