PS commands are not yet supported in this driver, when the driver for 2016 nano will GA the support for PS commands will also be implemented. for now you can change the registry values according to the WinOF User manual :
http://www.mellanox.com/related-docs/prod_software/MLNX_VPI_WinOF_User_Manual_v5.10.pdf
3.6.9.2 RoCE Options
The relevant registry key is in : HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\mlx4_bus\Parameters\Roce
The relevant parameter is roce_mode
The relevant value is 2 (RoCE v2