ASoC: mediatek: Make some symbols static

Fix sparse warnings:

sound/soc/mediatek/common/mtk-btcvsd.c:410:5: warning: symbol 'mtk_btcvsd_write_to_bt' was not declared. Should it be static?
sound/soc/mediatek/common/mtk-btcvsd.c:698:9: warning: symbol 'mtk_btcvsd_snd_read' was not declared. Should it be static?
sound/soc/mediatek/common/mtk-btcvsd.c:779:9: warning: symbol 'mtk_btcvsd_snd_write' was not declared. Should it be static?

Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Signed-off-by: Mark Brown <broonie@kernel.org>