ASoC: rsnd: use rsnd_src_convert_rate() once on rsnd_src_set_convert_rate_gen2()

using many rsnd_src_convert_rate() is not readable.

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