ASoC: mediatek: mt2701: fix non static symbol warning

Fixes the following sparse warning:

sound/soc/mediatek/mt2701/mt2701-afe-pcm.c:72:5: warning:
 symbol 'mt2701_dai_num_to_i2s' was not declared. Should it be static?
Signed-off-by: Wei Yongjun <yongjun_wei@trendmicro.com.cn>
Acked-by: Garlic Tseng <garlic.tseng@mediatek.com>
Signed-off-by: Mark Brown <broonie@kernel.org>

sound/soc/mediatek/mt2701/mt2701-afe-pcm.c

@@ -69,7 +69,7 @@ static const struct mt2701_afe_rate mt2701_afe_i2s_rates[] = {
 	{ .rate = 352800, .regvalue = 24 },
 };
 
-int mt2701_dai_num_to_i2s(struct mtk_base_afe *afe, int num)
+static int mt2701_dai_num_to_i2s(struct mtk_base_afe *afe, int num)
 {
 	int val = num - MT2701_IO_I2S;