ASoC: rsnd: add .fallback callback

Current R-Car sound has PIO fallback support if it couldn't use DMA.
This fallback is done in .remove callback, but, it should have
.fallback callback. Otherwise, normal .remove callback will have
strange behavior. This patch adds .fallback callback.

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