RDMA/nes: Make poll_cq return correct number of wqes during flush

When a flush request is given to the hw, it will place one cqe marked
as flushed (unless there is nothing to flush).  An application that is
waiting for all wqe's to complete will be left hanging.  This modifies
poll_cq to return the correct number of flushes for the pending
elements on the wq.

Signed-off-by: Don Wood <donald.e.wood@intel.com>
Signed-off-by: Roland Dreier <rolandd@cisco.com>