In sdma_tx_status() we must first find the current sdma_desc. In cyclic
mode we assume that this can always be found with vchan_find_desc().
This is true because do not remove the current descriptor from the
desc_issued list:

	/*
	 * Do not delete the node in desc_issued list in cyclic mode, otherwise
	 * the desc allocated will never be freed in vchan_dma_desc_free_list
	 */
	if (!(sdmac->flags & IMX_DMA_SG_LOOP))
		list_del(&vd->node);

We will change this in the next step, so check if the current descriptor is
the desired one also for the cyclic case.
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>
Link: https://lore.kernel.org/r/20191216105328.15198-9-s.hauer@pengutronix.deSigned-off-by: Vinod Koul <vkoul@kernel.org>