ASoC: rsnd: rename rsnd_scu_convert_rate_ctrl()

rsnd_scu_convert_rate_ctrl() is unclear naming,
and there is "rsnd_scu_convert_rate" variable.
These are confusable.
it renamed to rsnd_scu_set_convert_rate()

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