ASoC: mediatek: mt8183: make some functions static

Fixes the following sparse warnings:

sound/soc/mediatek/mt8183/mt8183-dai-i2s.c:966:5: warning:
 symbol 'mt8183_dai_i2s_get_share' was not declared. Should it be static?
sound/soc/mediatek/mt8183/mt8183-dai-i2s.c:986:5: warning:
 symbol 'mt8183_dai_i2s_set_priv' was not declared. Should it be static?

Fixes: a94aec03 ("ASoC: mediatek: mt8183: add platform driver")
Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Signed-off-by: Mark Brown <broonie@kernel.org>