ASoC: imx-ssi: Say if we fail to register a second AC'97 bus

Saves anyone wondering what happened if they run into this error.

Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
diff --git a/sound/soc/fsl/imx-ssi.c b/sound/soc/fsl/imx-ssi.c
index e174c17..3c520c4 100644
--- a/sound/soc/fsl/imx-ssi.c
+++ b/sound/soc/fsl/imx-ssi.c
@@ -560,6 +560,7 @@
 
 	if (ssi->flags & IMX_SSI_USE_AC97) {
 		if (ac97_ssi) {
+			dev_err(&pdev->dev, "AC'97 SSI already registered\n");
 			ret = -EBUSY;
 			goto failed_register;
 		}