net/mlx5: Update pfnum retrieval for devlink port attributes

Align mlx5 driver usage of 'pfnum' with the documentation clarification
introduced in commit bb70b0d4 ("devlink: Improve the port attributes
description").

Signed-off-by: Shay Drory <shayd@nvidia.com>
Reviewed-by: Mark Bloch <mbloch@nvidia.com>
Signed-off-by: Tariq Toukan <tariqt@nvidia.com>
Link: https://patch.msgid.link/1742392983-153050-4-git-send-email-tariqt@nvidia.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>