ASoC: rsnd: merge snd_pcm_ops::open and snd_soc_dai_ops::startup

Current rsnd driver has snd_pcm_ops::open and snd_soc_dai_ops::startup,
but, these are called from soc_pcm_open in same time.
This patch merge these into rsnd_soc_dai_startup()

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