ASoC: rsnd: tidyup rsnd_get_slot_xxx() naming

rsnd_get_slot_rdai() returns total slots (it returns 6 if total 6
channels) , and rsnd_get_slot_extend() returns extended SSI width
(it returns 8 if total 6 channels). This will be used on SSI multi
channel support too (It will return 2 if total 6 channels with 3 SSI).
But, it is using confusable naming.
This patch changes rsnd_get_slot_rdai() -> rsnd_get_slot(),
rsnd_get_slot_extend() -> rsnd_get_slot_width()

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