See here:
http://community.mellanox.com/docs/DOC-1332#comment-1187
In short: uninstall drivers that came with ESXi 5.5, install "old" 1.8.2 drivers from Mellanox.
See here:
http://community.mellanox.com/docs/DOC-1332#comment-1187
In short: uninstall drivers that came with ESXi 5.5, install "old" 1.8.2 drivers from Mellanox.