ASoC: rsnd: add rsnd_mod_next() for for_each_rsnd_mod_xxx()

Current rsnd driver is using too complex macro for for-loop of each mod.
In order to simplify this issue, this patch adds new rsnd_mod_next()
which is non-macro.

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