scsi: hisi_sas: Modify return type of debugfs functions

For functions which always return 0, which is never checked, make to return
void.

Signed-off-by: Luo Jiaxing <luojiaxing@huawei.com>
Signed-off-by: John Garry <john.garry@huawei.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>