net/mlx5: Introduce physical port PFC access functions

Add access functions to set and query a physical port PFC
(Priority Flow Control) parameters.

Signed-off-by: Achiad Shochat <achiad@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>