RDMA/hns: Modify uar allocation algorithm to avoid bitmap exhaust

This patch modified uar allocation algorithm in hns_roce_uar_alloc
function to avoid bitmap exhaust.

Signed-off-by: Wei Hu (Xavier) <xavier.huwei@huawei.com>
Reviewed-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>