ASoC: rsnd: more clear rsnd_get_dalign() for DALIGN

On Renesas sound device, DALIGN which exchanges channel position
is needed because SW and HW are using defferent data order if
16bit data. It is not needed when 24bit data.
rsnd_get_dalign() returns necessary value, but it was confusable
code. This patch makes it more simple.

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>