ASoC: soc-pcm: add soc_rtd_prepare()

Add soc_rtd_prepare() to make the code easier to read

Signed-off-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Link: https://lore.kernel.org/r/875zh4mi9v.wl-kuninori.morimoto.gx@renesas.com


Reviewed-by: Ranjani Sridharan <ranjani.sridharan@linux.intel.com>
Reviewed-by: Pierre-Louis Bossart <pierre-louis.bossart@linux.intel.com>
Signed-off-by: Mark Brown <broonie@kernel.org>