RDMA/cxgb4: Mark QP in error before disabling the queue in firmware

QPs need to be moved to error before telling the firwmare to shutdown
the queue.  Otherwise, the application can submit WRs that will never
get fetched by the hardware and never flushed by the driver.

Signed-off-by: Kumar Sanghvi <kumaras@chelsio.com>
Acked-by: Steve Wise <swsie@opengridcomputing.com>
Signed-off-by: Roland Dreier <roland@purestorage.com>