IB/mthca: Add modify_device method to set node description

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: Michael S. Tsirkin <mst@mellanox.co.il>
Signed-off-by: Roland Dreier <rolandd@cisco.com>