IB/mthca: Use correct GID in MADs sent on port 2

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