ASoC: rsnd: add rsnd_src_get_in/out_rate()

SRC will convert rate, and then, CMD and SSI want to know its
rate (= SRC.in / SRC.out) for each purpose.
Current driver is supporting only Playback, but SRC+Capture support
needs more flexibility.
This patch adds rsnd_src_get_in/out_rate() for it.

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