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

It makes no sense keeping an 'in_use' flag when the multiple descriptor
mechanism already prevents a new DMA transfer to be issued when another
one is in course.

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>