ASoC: soc-dai: don't overwide dai->driver->ops

Current ASoC overwrites null_dai_ops to dai->driver->ops if it
was NULL. But, we can remove it if framework always checks
dai->driver->ops when it uses DAI callbacks.
This patch do it, and removes null_dai_ops.

Signed-off-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Reviewed-By: Ranjani Sridharan <ranjani.sridharan@linux.intel.com>
Link: https://lore.kernel.org/r/87zhb1sskc.wl-kuninori.morimoto.gx@renesas.com


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