RDMA/cxgb3: QP flush fixes

- Flush the QP only after the HW disables the connection.  Currently
  we flush the QP when transitioning to CLOSING.  This exposes a race
  condition where the HW can complete a RECV WR, for instance, -and-
  the SW can flush that same WR.

- Only call CQ event handlers on flush IFF we actually flushed something.

Signed-off-by: Steve Wise <swise@opengridcomputing.com>
Signed-off-by: Roland Dreier <rolandd@cisco.com>