dmaengine: imx-dma: remove 'resbytes' field of 'internal' structure.

Use per-descriptor 'len' field to keep track of the remaining bytes
instead. This goes on the direction of eventually removing the
'internal' structure.

Signed-off-by: Javier Martin <javier.martin@vista-silicon.com>
Acked-by: Sascha Hauer <s.hauer@pengutronix.de>
Signed-off-by: Vinod Koul <vinod.koul@linux.intel.com>