mmc: owl-mmc: Use dma_request_chan() instead dma_request_slave_channel()

dma_request_slave_channel() is a wrapper on top of dma_request_chan()
eating up the error code.

By using dma_request_chan() directly the driver can support deferred
probing against DMA.
Signed-off-by: Peter Ujfalusi <peter.ujfalusi@ti.com>
Link: https://lore.kernel.org/r/20191217112934.31535-1-peter.ujfalusi@ti.comSigned-off-by: Ulf Hansson <ulf.hansson@linaro.org>

drivers/mmc/host/owl-mmc.c

@@ -616,10 +616,10 @@ static int owl_mmc_probe(struct platform_device *pdev)
 
 	pdev->dev.coherent_dma_mask = DMA_BIT_MASK(32);
 	pdev->dev.dma_mask = &pdev->dev.coherent_dma_mask;
-	owl_host->dma = dma_request_slave_channel(&pdev->dev, "mmc");
-	if (!owl_host->dma) {
+	owl_host->dma = dma_request_chan(&pdev->dev, "mmc");
+	if (IS_ERR(owl_host->dma)) {
 		dev_err(owl_host->dev, "Failed to get external DMA channel.\n");
-		ret = -ENXIO;
+		ret = PTR_ERR(owl_host->dma);
 		goto err_free_host;
 	}