RDMA/hw/hns/hns_roce_mr: Add missing description for 'hr_dev' param

RDMA/hw/hns/hns_roce_mr: Add missing description for 'hr_dev' param

Fixes the following W=1 kernel build warning(s):

 drivers/infiniband/hw/hns/hns_roce_mr.c:1003: warning: Function parameter or member 'hr_dev' not described in 'hns_roce_mtr_create'

Link: https://lore.kernel.org/r/20210121094519.2044049-6-lee.jones@linaro.org
Cc: Lijun Ou <oulijun@huawei.com>
Cc: Weihang Li <liweihang@huawei.com>
Cc: Doug Ledford <dledford@redhat.com>
Cc: Jason Gunthorpe <jgg@ziepe.ca>
Cc: linux-rdma@vger.kernel.org
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>