RDMA/hns: Clean up the return value check of hns_roce_alloc_cmd_mailbox()

hns_roce_alloc_cmd_mailbox() never returns NULL, so the check should be
IS_ERR(). And the error code should be converted as the function's return
value.

Link: https://lore.kernel.org/r/20220302064830.61706-8-liangwenpeng@huawei.com


Signed-off-by: Wenpeng Liang <liangwenpeng@huawei.com>
Reviewed-by: Leon Romanovsky <leonro@nvidia.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>