Make sure that we call the dma_unmap_sg on the correct scatterlist on
completion with the correct sg_nents.

Use sg_table to managed the DMA mapping and at the same time add the needed
dma_sync calls for the sg_table.
Signed-off-by: Peter Ujfalusi <peter.ujfalusi@ti.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>