ASoC: soc-core: remove null_snd_soc_ops

ASoC: soc-core: remove null_snd_soc_ops

All rtd->dai_link callback functions are controlled by soc_rtd_xxxx(),
and checking rtd->dai_link->ops.
We don't need to have null_snd_soc_ops anymore.
This patch removes it.

Signed-off-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Link: https://lore.kernel.org/r/87zhegl3oz.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>