ASoC: fsl_asrc_dma: fix potential null-ptr-deref

dma_request_slave_channel() may return NULL which will lead to
NULL pointer dereference error in 'tmp_chan->private'.

Correct this behaviour by, first, switching from deprecated function
dma_request_slave_channel() to dma_request_chan(). Secondly, enable
sanity check for the resuling value of dma_request_chan().
Also, fix description that follows the enacted changes and that
concerns the use of dma_request_slave_channel().

Fixes: 706e2c88 ("ASoC: fsl_asrc_dma: Reuse the dma channel if available in Back-End")
Co-developed-by: Natalia Petrova <n.petrova@fintech.ru>
Signed-off-by: Nikita Zhandarovich <n.zhandarovich@fintech.ru>
Acked-by: Shengjiu Wang <shengjiu.wang@gmail.com>
Link: https://lore.kernel.org/r/20230417133242.53339-1-n.zhandarovich@fintech.ruSigned-off-by: Mark Brown <broonie@kernel.org>

sound/soc/fsl/fsl_asrc_dma.c

@@ -209,14 +209,19 @@ static int fsl_asrc_dma_hw_params(struct snd_soc_component *component,
 		be_chan = soc_component_to_pcm(component_be)->chan[substream->stream];
 		tmp_chan = be_chan;
 	}
-	if (!tmp_chan)
-		tmp_chan = dma_request_slave_channel(dev_be, tx ? "tx" : "rx");
+	if (!tmp_chan) {
+		tmp_chan = dma_request_chan(dev_be, tx ? "tx" : "rx");
+		if (IS_ERR(tmp_chan)) {
+			dev_err(dev, "failed to request DMA channel for Back-End\n");
+			return -EINVAL;
+		}
+	}
 
 	/*
 	 * An EDMA DEV_TO_DEV channel is fixed and bound with DMA event of each
 	 * peripheral, unlike SDMA channel that is allocated dynamically. So no
 	 * need to configure dma_request and dma_request2, but get dma_chan of
-	 * Back-End device directly via dma_request_slave_channel.
+	 * Back-End device directly via dma_request_chan.
 	 */
 	if (!asrc->use_edma) {
 		/* Get DMA request of Back-End */