dmaengine: imx-dma: remove dma_mode member of internal structure.

dmaengine now provides 'enum dma_transfer_direction' to properly
specify DMA transfer direction. For this reason, DMA_MODE_* defines
are replaced by this new type and therefore dma_mode member becomes
redundant.

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>