drm/i2c/tda998x: Make tda998x_audio_digital_mute static

Fix the following sparse warning:

drivers/gpu/drm/i2c/tda998x_drv.c:1136:5: warning:
symbol 'tda998x_audio_digital_mute' was not declared. Should it be static?
Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Samuel Zou <zou_wei@huawei.com>
Cc: Russell King <linux@armlinux.org.uk>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/1588819768-11818-1-git-send-email-zou_wei@huawei.com

drivers/gpu/drm/i2c/tda998x_drv.c

@@ -1133,7 +1133,8 @@ static void tda998x_audio_shutdown(struct device *dev, void *data)
 	mutex_unlock(&priv->audio_mutex);
 }
 
-int tda998x_audio_digital_mute(struct device *dev, void *data, bool enable)
+static int tda998x_audio_digital_mute(struct device *dev, void *data,
+				      bool enable)
 {
 	struct tda998x_priv *priv = dev_get_drvdata(dev);