ASoC: soc-pcm: makes snd_soc_dpcm_can_be_xxx() local

Merge series from Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>:

No driver is calling snd_soc_dpcm_can_be_xxx() functions. We don't need
to have EXPORT_SYMBOL_GPL() for them. This patch-set makes it static function.