drm/bridge: sii902x: Make sii902x_audio_digital_mute static

Fix sparse warning:

drivers/gpu/drm/bridge/sii902x.c:665:5: warning:
 symbol 'sii902x_audio_digital_mute' was not declared. Should it be static?
Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Signed-off-by: Andrzej Hajda <a.hajda@samsung.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190614153623.28708-1-yuehaibing@huawei.com

drivers/gpu/drm/bridge/sii902x.c

@@ -662,7 +662,8 @@ static void sii902x_audio_shutdown(struct device *dev, void *data)
 	clk_disable_unprepare(sii902x->audio.mclk);
 }
 
-int sii902x_audio_digital_mute(struct device *dev, void *data, bool enable)
+static int sii902x_audio_digital_mute(struct device *dev,
+				      void *data, bool enable)
 {
 	struct sii902x *sii902x = dev_get_drvdata(dev);