Add a modify_device method to mthca, which implements setting the node
description.  This makes the writable "node_desc" sysfs attribute work
for Mellanox HCAs.
Signed-off-by: NMichael S. Tsirkin <mst@mellanox.co.il>
Signed-off-by: NRoland Dreier <rolandd@cisco.com>