ASoC: rsnd: rename "slots" related variable

Current Renesas sound driver has slots and slots_num in
struct rsnd_dai, but these are very un-understandable naming
(It had named from TDM slots).
In this driver, the "slots" means total usable channels, and
"stot_num" means SSI lane number if Multi SSI was used.
To more understandable code, this patch renames "slots" to
"max_channels", and "slots_num" to "ssi_lane", and replaces related
functions name.

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