spi: mxs: 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/20191212135550.4634-6-peter.ujfalusi@ti.comSigned-off-by: Mark Brown <broonie@kernel.org>

drivers/spi/spi-mxs.c

@@ -590,10 +590,10 @@ static int mxs_spi_probe(struct platform_device *pdev)
 	if (ret)
 		goto out_master_free;
 
-	ssp->dmach = dma_request_slave_channel(&pdev->dev, "rx-tx");
-	if (!ssp->dmach) {
+	ssp->dmach = dma_request_chan(&pdev->dev, "rx-tx");
+	if (IS_ERR(ssp->dmach)) {
 		dev_err(ssp->dev, "Failed to request DMA\n");
-		ret = -ENODEV;
+		ret = PTR_ERR(ssp->dmach);
 		goto out_master_free;
 	}