ASoC: soc-utils: add snd_soc_dlc_is_dummy()

We have snd_soc_xxx_is_dummy() functions, but not for dlc.
Let's add it.

Signed-off-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Link: https://patch.msgid.link/87cydc8vup.wl-kuninori.morimoto.gx@renesas.com


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