scsi: hisi_sas: Remove unnecessary variable to hold DMA map elements

Use slot->n_elem to store the return value of dma_map_sg() for SSP and SMP
IOs, and remove unnecessary variable n_elem_req.

Link: https://lore.kernel.org/r/1657823002-139010-3-git-send-email-john.garry@huawei.com


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>