scsi: hisi_sas: fix v2 hw underflow residual value

The value dw0 is the residual bytes when UNDERFLOW error happens, but we
filled the residual with the value of dw3 before. So change the residual
from dw3 to dw0.

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>