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.comSigned-off-by: NWenpeng Liang <liangwenpeng@huawei.com>
Reviewed-by: NLeon Romanovsky <leonro@nvidia.com>
Signed-off-by: NJason Gunthorpe <jgg@nvidia.com>