scsi: hisi_sas: relocate some code to decrease cyclomatic complexity of function hisi_sas_task_prep()

Relocate the codes related to dma_map/unmap to decrease cyclomatic
complexity of function hisi_sas_task_prep().
Signed-off-by: NXiang Chen <chenxiang66@hisilicon.com>
Signed-off-by: NJohn Garry <john.garry@huawei.com>