ASoC: soc-core: tidyup soc_new_pcm_runtime() rtd setups

soc_new_pcm_runtime() setups rtd, but code is very random.
This patch tidyup it.

Signed-off-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Tested-by: Pierre-Louis Bossart <pierre-louis.bossart@linux.intel.com>
Link: https://lore.kernel.org/r/87ftdqbokh.wl-kuninori.morimoto.gx@renesas.com


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