ASoC: cygnus: Remove support for 8 bit audio and for mono

These modes of operation were not working properly and it is
unclear if the hardware could fully support these modes properly.
There is little to be gained by enabling these modes, therefore,
we will just remove support.

Signed-off-by: Lori Hikichi <lori.hikichi@broadcom.com>
Signed-off-by: Mark Brown <broonie@kernel.org>