ASoC: rsnd: add rsnd_adg_set_ssi_clk() and cleanup adg

This patch adds rsnd_adg_set_ssi_clk() to access to
AUDIO_CLK_SEL0/1/2, and removes last user of
rsnd_write/read/bset which is very low level function.

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