dmaengine: Clarify device parameter for dma_sync_*_for_*()

   The peripheral driver is expected to have mapped the scatterlist for

   the DMA operation prior to calling device_prep_slave_sg, and must

   keep the scatterlist mapped until the DMA operation has completed.

   The scatterlist must be mapped using the DMA struct device.  So,

   normal setup should look like this:

   The scatterlist must be mapped using the DMA struct device.

   If a mapping needs to be synchronized later, dma_sync_*_for_*() must be

   called using the DMA struct device, too.

   So, normal setup should look like this:

	nr_sg = dma_map_sg(chan->device->dev, sgl, sg_len);

	if (nr_sg == 0)