There seems to be a driver that might be able to work with 8.1 client :
http://www.mellanox.com/downloads/WinOF/MLNX_VPI_WinOF-4_95_All_win2012R2_x64.exe
I would go through the archives and install an older version in case that doesn't work. Remember that if you don't have a managed switch you need 1 (or both) ports to run an instance of OpenSM that will manage the connections.
http://www.mellanox.com/related-docs/prod_software/MLNX_VPI_WinOF_User_Manual_v4.95.pdf