ASoC: rsnd: merge rsnd_scu_start/stop() and rsnd_scu_transfer_start/stop()

rsnd_scu_transfer_start/stop() are no longer needed.
merge into rsnd_scu_start/stop()

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