dmaengine: imx-dma: remove internal structure.

This structure was created to allow an smoothless
merge but was meant to be removed. Remaining members
'hw_chaining' and 'watchdog' are moved to the channel
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>