mmc: mxcmmc: 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/20191217112803.31161-1-peter.ujfalusi@ti.comSigned-off-by: Ulf Hansson <ulf.hansson@linaro.org>

drivers/mmc/host/mxcmmc.c

@@ -1121,7 +1121,16 @@ static int mxcmci_probe(struct platform_device *pdev)
 	mxcmci_writel(host, host->default_irq_mask, MMC_REG_INT_CNTR);
 
 	if (!host->pdata) {
-		host->dma = dma_request_slave_channel(&pdev->dev, "rx-tx");
+		host->dma = dma_request_chan(&pdev->dev, "rx-tx");
+		if (IS_ERR(host->dma)) {
+			if (PTR_ERR(host->dma) == -EPROBE_DEFER) {
+				ret = -EPROBE_DEFER;
+				goto out_clk_put;
+			}
+
+			/* Ignore errors to fall back to PIO mode */
+			host->dma = NULL;
+		}
 	} else {
 		res = platform_get_resource(pdev, IORESOURCE_DMA, 0);
 		if (res) {