ASoC: rsnd: don't use runtime->sample_bits

Current rsnd driver is judging 16bit/24bit data by using
runtime->sample_bits, but it is indicating physical size,
not format size. This is confusable code.
This patch uses snd_pcm_format_width() to be more correct code.

Tested-by: Hiroyuki Yokoyama <hiroyuki.yokoyama.vx@renesas.com>
Signed-off-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Signed-off-by: Mark Brown <broonie@kernel.org>