ASoC: mediatek: export mtk-afe symbols as needed

The new mt6797-afe driver uses some functions in a common file, which
works for a built-in driver but fails for a loadable module:

ERROR: "mtk_afe_pcm_free" [sound/soc/mediatek/mt6797/snd-soc-mt6797-afe.ko] undefined!
ERROR: "mtk_afe_add_sub_dai_control" [sound/soc/mediatek/mt6797/snd-soc-mt6797-afe.ko] undefined!
ERROR: "mtk_afe_pcm_new" [sound/soc/mediatek/mt6797/snd-soc-mt6797-afe.ko] undefined!
ERROR: "mtk_afe_combine_sub_dai" [sound/soc/mediatek/mt6797/snd-soc-mt6797-afe.ko] undefined!
ERROR: "mtk_afe_pcm_ops" [sound/soc/mediatek/mt6797/snd-soc-mt6797-afe.ko] undefined!

This exports the five symbols above for modules.

Fixes: b3c702f5 ("ASoC: mt6797: combine DAI to register component")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Mark Brown <broonie@kernel.org>