ASoC: rsnd: add rsnd_mod_get() macro and use it

Renesas sound driver has SSI/SRC/DVC/CTU/MIX, and these are controlled
as modules. And these module are member of each modules's private data.
It used own method to get module pointer, but Let's use common method

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