ASoC: rsnd: remove rsnd_priv_read/write/bset()

adg.c only used rsnd_priv_read/write/bset()
which is the only user of NULL mod.
but, it can be removed.

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