ASoC: fsl_sai: Make Synchronous and Asynchronous modes exclusive

The previous patch (ASoC: fsl_sai: Add asynchronous mode support) added
new Device Tree bindings for Asynchronous and Synchronous modes support.
However, these two shall not be present at the same time.

So this patch just simply makes them exclusive so as to avoid incorrect
Device Tree binding usage.
Signed-off-by: NNicolin Chen <nicoleotsuka@gmail.com>
Signed-off-by: NMark Brown <broonie@linaro.org>

Documentation/devicetree/bindings/sound/fsl-sai.txt

@@ -38,8 +38,7 @@ Note:
   default synchronous mode (sync Rx with Tx) will be used, which means both
   transimitter and receiver will send and receive data by following clocks
   of transimitter.
-- fsl,sai-asynchronous will be ignored if fsl,sai-synchronous-rx property is
-  already present.
+- fsl,sai-asynchronous and fsl,sai-synchronous-rx are exclusive.
 
 Example:
 sai2: sai@40031000 {

sound/soc/fsl/fsl_sai.c

@@ -634,6 +634,13 @@ static int fsl_sai_probe(struct platform_device *pdev)
 	fsl_sai_dai.symmetric_channels = 1;
 	fsl_sai_dai.symmetric_samplebits = 1;
 
+	if (of_find_property(np, "fsl,sai-synchronous-rx", NULL) &&
+	    of_find_property(np, "fsl,sai-asynchronous", NULL)) {
+		/* error out if both synchronous and asynchronous are present */
+		dev_err(&pdev->dev, "invalid binding for synchronous mode\n");
+		return -EINVAL;
+	}
+
 	if (of_find_property(np, "fsl,sai-synchronous-rx", NULL)) {
 		/* Sync Rx with Tx */
 		sai->synchronous[RX] = false;