dmaengine: imx-dma: remove sg member from internal structure.

This member is redundant, because it is already present in
descriptor structure. Removing it will make further removing
of 'internal' structure easier.

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>