ASoC: rcar ctu: Staticise local symbols

rsnd_of_parse_ctu() is not used outside this file so it can be static.
Fixes the following sparse warning:

	sound/soc/sh/rcar/ctu.c:72:6: warning: symbol 'rsnd_of_parse_ctu' was
		not declared. Should it be static?

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Acked-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Signed-off-by: Mark Brown <broonie@kernel.org>