dmaengine: dmatest: Use dmaengine_get_dma_device

By using the dmaengine_get_dma_device() to get the device for
dma_api use, the dmatest can support per channel coherency if it is
supported by the DMA controller.

Signed-off-by: Peter Ujfalusi <peter.ujfalusi@ti.com>
Link: https://lore.kernel.org/r/20201208090440.31792-11-peter.ujfalusi@ti.com
Signed-off-by: Vinod Koul <vkoul@kernel.org>