Shaun Slasor - If ibv_devinfo doesn't show the PSID for some reason, another option is to use an older release of the Mellanox Firmware Tools. Something before the 3.x series.
Version 2.7.2 is still available on the old Mellanox website here:
http://old.mellanox.com/content/pages.php?pg=management_tools&menu_section=34