scsi: ufs: core: Split ufshcd_map_sg()

Take out the "map scatter-gather list to prdt" part of the code in
ufshcd_map_sg() and split it into a new function ufshcd_sgl_to_prdt().

Signed-off-by: Bean Huo <beanhuo@micron.com>
Reviewed-by: Avri Altman <avri.altman@wdc.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>