FROMGIT: 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().

Change-Id: Ie16b9cb0008133d04f29647d6f07751dd90d5dc8
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>
(cherry picked from commit 7a4df79d git://git.kernel.org/pub/scm/linux/kernel/git/mkp/scsi.git

 for-next)
Signed-off-by: Bart Van Assche <bvanassche@google.com>