mthca_create_ah() includes the port number in the GID index. The reverse
needs to be done in mthca_read_ah().

Noted by Hal Rosenstock.
Signed-off-by: NMichael S. Tsirkin <mst@mellanox.co.il>
Signed-off-by: NRoland Dreier <rolandd@cisco.com>