RDMA/hns: Fix the bug with NULL pointer

When the last QP of eight QPs is not exist in
hns_roce_v1_mr_free_work_fn function, the
print for qpn of hr_qp may introduce a
calltrace for NULL pointer.

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