scsi: hisi_sas: Relocate some code to reduce complexity

Relocate the codes related to dma_map/unmap in hisi_sas_task_prep() to
reduce complexity, with a view to add DIF/DIX support.

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>