RDMA/hns: Fix the wrong type of parameter "op" of the mailbox

The "op" field of the mailbox occupies 8 bits, so the parameter "op"
should be of type u8.

Link: https://lore.kernel.org/r/20220302064830.61706-5-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>