ASoC: renesas: rsnd: allow to use ADG as standalone

Audio clock generator (= ADG) can be used standalone (without DAI),
but current driver indicates error if it doesn't have DAI on DT.
It is not error, allow to use ADG as standalone.

Signed-off-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Link: https://patch.msgid.link/87v7r22x0a.wl-kuninori.morimoto.gx@renesas.com


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