RDMA/hns: Only assign dqpn if IB_QP_PATH_DEST_QPN bit is set

Only when the IB_QP_PATH_DEST_QPN flag of attr_mask is set
is it valid to assign the dqpn field of qp context

Signed-off-by: Lijun Ou <oulijun@huawei.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>