scsi: hisi_sas: make return type of prep functions void

Since the task prep functions now should not fail, adjust the return
types to void.

In addition, some checks in the task prep functions are relocated to the
main module; this is specifically the check for the number of elements
in an sg list exceeded the HW SGE limit.

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>