RDMA/bnxt_re: Flush CQ notification Work Queue before destroying QP

Destroy_qp shall wait for any outstanding CQ notification to be
flushed out before proceeding with QP destroy. Flushing the WQ
before destroying the QP.

Signed-off-by: Selvin Xavier <selvin.xavier@broadcom.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>