IB/mthca: Fix offset in query_gid method

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: Roland Dreier <rolandd@cisco.com>