IB/mthca: don't change driver's copy of attributes if modify QP fails

Only change the driver's copy of the QP attributes in modify QP after
checking the modify QP command completed successfully.

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