ASoC: add snd_soc_get_stream_cpu()

We are using get_stream_cpu() to get CPU stream which cares
Codec2Codec. But it is static function for now, and we want to use it
from other files. This patch makes it global function.

Signed-off-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Link: https://lore.kernel.org/r/87fs7cj9mf.wl-kuninori.morimoto.gx@renesas.com


Signed-off-by: Mark Brown <broonie@kernel.org>