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>