GuidInfo records have 8 byte GUIDs in them, so an index should be
multiplied by 8 to get an offset.  mthca_query_gid() was incorrectly
multiplying by 16.

Noticed by Leonid Keller <leonid@mellanox.co.il>.
Signed-off-by: NRoland Dreier <rolandd@cisco.com>