scsi: hisi_sas: Relocate some codes to avoid an unused check

scsi: hisi_sas: Relocate some codes to avoid an unused check

In function hisi_sas_task_prep(), we check asd_sas_port, but in function
hisi_sas_task_exec(), we already refer to asd_sas_port by using function
dev_to_hisi_hba() implicitly. So to avoid this possible invalid
dereference, relocate the check to function hisi_sas_task_prep().

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>