ASoC: rsnd: add rsnd_io_to_rdai()

Current rsnd driver request rdai in everywhere, but each mod should
reach to rdai by itself. this patch adds missing rsnd_io_to_rdai().

io   = rsnd_mod_to_io(mod);
rdai = rsnd_io_to_rdai(io);

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