ASoC: Fix build error in sst_platform.c

Fix below build error:

CC sound/soc/mid-x86/sst_platform.o
sound/soc/mid-x86/sst_platform.c: In function 'sst_platform_open':
sound/soc/mid-x86/sst_platform.c:274:2: error: 'ret_val' undeclared (first use in this function) sound/soc/mid-x86/sst_platform.c:274:2: note: each undeclared identifier is reported only once for each function it appears in make[3]: *** [sound/soc/mid-x86/sst_platform.o] Error 1
make[2]: *** [sound/soc/mid-x86] Error 2
make[1]: *** [sound/soc] Error 2
make: *** [sound] Error 2

---
 sound/soc/mid-x86/sst_platform.c |    1 +
 1 files changed, 1 insertions(+), 0 deletions(-)

index 24f9471..11c39c5 100644
@@ -267,6 +267,7 @@ static int sst_platform_open(struct snd_pcm_substream *substream)
 {
 	struct snd_pcm_runtime *runtime = substream->runtime;
 	struct sst_runtime_stream *stream;
+	int ret_val;
 
 	pr_debug("sst_platform_open called\n");
 

1.7.5.4