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