scsi: hisi_sas: Disable stash for v3 hw

For v3 hw, stash is enabled to promote performance, but it does little to
improve performance according to current tests. What's more, it causes
exceptions for some situations, so disable it.

Signed-off-by: Xiang Chen <chenxiang66@hisilicon.com>
Signed-off-by: John Garry <john.garry@huawei.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>