ASoC: rsnd: add rsnd_scu_init(), and separate init/start

Current scu.c has rsnd_scu_start(),
and, operation of initialization/start
are implemented in this function.
This patch adds new rsnd_scu_init() and separates
rsnd_scu_start(), since rsnd_mod_ops has .init/.start callbacks.

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