ASoC: rsnd: remove rsnd_get_slot_runtime()

Current Renesas sound driver is using rsnd_get_slot_runtime(), but
it is same as runtime->channels. This patch removes
rsnd_get_slot_runtime()

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