ASoC: rsnd: remove duplicate *priv from rsnd_dma

*priv pointer exists under rsnd_mod,
and, it can get rsnd_mod pointer from rsnd_dma.
remove duplicate rsnd_dma :: priv

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