Add one field to save the transfer direction for struct sprd_dma_desc,
which is used to get correct transfer residue depending on the transfer
direction.

[Baolin Wang adds one field to present the transfer direction]
Signed-off-by: Eric Long <eric.long@spreadtrum.com>
Signed-off-by: Baolin Wang <baolin.wang@linaro.org>
Signed-off-by: Vinod Koul <vkoul@kernel.org>