mmc: bcm2835: 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>
Reviewed-by: Nicolas Saenz Julienne <nsaenzjulienne@suse.de>
Link: https://lore.kernel.org/r/20191217122254.7103-1-peter.ujfalusi@ti.comSigned-off-by: Ulf Hansson <ulf.hansson@linaro.org>

drivers/mmc/host/bcm2835.c

@@ -1393,7 +1393,17 @@ static int bcm2835_probe(struct platform_device *pdev)
 	host->dma_chan = NULL;
 	host->dma_desc = NULL;
 
-	host->dma_chan_rxtx = dma_request_slave_channel(dev, "rx-tx");
+	host->dma_chan_rxtx = dma_request_chan(dev, "rx-tx");
+	if (IS_ERR(host->dma_chan_rxtx)) {
+		ret = PTR_ERR(host->dma_chan_rxtx);
+		host->dma_chan_rxtx = NULL;
+
+		if (ret == -EPROBE_DEFER)
+			goto err;
+
+		/* Ignore errors to fall back to PIO mode */
+	}
+
 
 	clk = devm_clk_get(dev, NULL);
 	if (IS_ERR(clk)) {