ASoC: rsnd: care snd_kcontrol's index

rsnd might be used in multi-codec sound card.
Then, same name kcontrol will be registered many times, and it will
be error. This patch fixes this issue by using .index

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