ASoC: broadwell: add channel constraint

BDW boards using this machine driver supports only stereo capture and
playback. Implement a constraint to enforce it.

Signed-off-by: Brent Lu <brent.lu@intel.com>
Acked-by: Pierre-Louis Bossart <pierre-louis.bossart@linux.intel.com>
Link: https://lore.kernel.org/r/1588007614-25061-4-git-send-email-brent.lu@intel.com


Signed-off-by: Mark Brown <broonie@kernel.org>