RDMA/hns: Fix a wrong judgment of return value

hns_roce_alloc_mtt_range() never return -1, ret should be checked
whether it is zero instead of -1.

Fixes: 1ceb0b11 ("RDMA/hns: Fix non-standard error codes")
Link: https://lore.kernel.org/r/1584674622-52773-3-git-send-email-liweihang@huawei.com


Signed-off-by: Weihang Li <liweihang@huawei.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>