drm/bridge: sii902x: fix missing static

Make sii902x_audio_digital_mute() static as it's used only in the single
file.

Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>

drivers/gpu/drm/bridge/sii902x.c

@@ -662,7 +662,7 @@ static void sii902x_audio_shutdown(struct device *dev, void *data)
 	}
 }
 
-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);