IB/mlx5: Enable MAD_IFC commands for IB ports only

MAD_IFC command is supported only for physical functions (PF)
and when physical port is IB. The proposed fix enforces it.

Fixes: d603c809

 ("IB/mlx5: Fix decision on using MAD_IFC")
Reported-by: David Chang <dchang@suse.com>
Signed-off-by: Noa Osherovich <noaos@mellanox.com>
Signed-off-by: Leon Romanovsky <leon@kernel.org>
Signed-off-by: Doug Ledford <dledford@redhat.com>