scsi: hisi_sas: Don't bother clearing status buffer IU in task prep

For struct hisi_sas_status_buffer, it contains struct hisi_sas_err_record
and iu[1024]. The struct iu[1024] will be filled fully by the response of
disks, so it is not need to initialize them to 0, but for the struct
hisi_sas_err_record, SAS controller only fill some fields of
hisi_sas_err_record according to hw designer, so it should be initialised
to 0.  After the change, cpu utilization percentage of memset() is changed
from 1.7% to 0.12%.

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>