greybus: audio: fix some sparse static warnings

This makes some functions and structures static, as warned by sparse, as
they don't need to be global.

Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>
Reviewed-by: Alex Elder <elder@linaro.org>
diff --git a/drivers/staging/greybus/audio-pcm.c b/drivers/staging/greybus/audio-pcm.c
index 28b5e11..9243630 100644
--- a/drivers/staging/greybus/audio-pcm.c
+++ b/drivers/staging/greybus/audio-pcm.c
@@ -235,7 +235,7 @@
 			PREALLOC_BUFFER, PREALLOC_BUFFER_MAX);
 }
 
-struct snd_soc_platform_driver gb_soc_platform = {
+static struct snd_soc_platform_driver gb_soc_platform = {
 	.ops		= &gb_pcm_ops,
 	.pcm_new	= gb_pcm_new,
 	.pcm_free	= gb_pcm_free,