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>

sound/soc/mediatek/common/mtk-btcvsd.c

@@ -407,7 +407,7 @@ static int mtk_btcvsd_read_from_bt(struct mtk_btcvsd_snd *bt,
 	return 0;
 }
 
-int mtk_btcvsd_write_to_bt(struct mtk_btcvsd_snd *bt,
+static int mtk_btcvsd_write_to_bt(struct mtk_btcvsd_snd *bt,
 				  enum bt_sco_packet_len packet_type,
 				  unsigned int packet_length,
 				  unsigned int packet_num,
@@ -695,7 +695,7 @@ static int wait_for_bt_irq(struct mtk_btcvsd_snd *bt,
 	return 0;
 }
 
-ssize_t mtk_btcvsd_snd_read(struct mtk_btcvsd_snd *bt,
+static ssize_t mtk_btcvsd_snd_read(struct mtk_btcvsd_snd *bt,
 				   char __user *buf,
 				   size_t count)
 {
@@ -776,7 +776,7 @@ ssize_t mtk_btcvsd_snd_read(struct mtk_btcvsd_snd *bt,
 	return read_count;
 }
 
-ssize_t mtk_btcvsd_snd_write(struct mtk_btcvsd_snd *bt,
+static ssize_t mtk_btcvsd_snd_write(struct mtk_btcvsd_snd *bt,
 				    char __user *buf,
 				    size_t count)
 {