ASoC: rsnd: remove 16ch support for now

R-Car Gen3 sound device can use TDM 16ch, but its usage is very
limited. It is not yet supported.
Let's remove unexpected/unsupported channel settings for now.
This patch makes xx_ADINR register settings sanity without
runtime check.

R-Car Gen3 datasheet

40.4.7 Note on Usage of the SSIU Module
When an SSIU module is used in "TDM-16ch mode",
only the transfer with the external memory is available.

Reported-by: Nguyen Viet Dung <dung.nguyen.aj@renesas.com>
Signed-off-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Signed-off-by: Mark Brown <broonie@kernel.org>