ASoC: add null_snd_soc_ops and reduce NULL ops check

Double NULL pointer check for ops and ops->func is difficult to read
and might be forget to check it if new func was add.
This patch adds new null_snd_soc_ops and use it if rtd->dai_link didn't
have it to avoid NULL ops, and reduces ops NULL check.

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