ASoC: soc-core: don't use codec_dais on soc_bind_dai_link()

Current soc_bind_dai_link() is substituting rtd->codec_dais to
codec_dais, and sets found DAI into it.
But, it is a little bit un-readable / un-understandable to
know detail of rtd, and it will make difficult to understand
rtd->cpu_dais if Multi CPU was supported.
This patch cleanup it and prepare for Multi CPU support.

Signed-off-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Signed-off-by: Mark Brown <broonie@kernel.org>