ASoC: Intel: bxt: Use refcap device for mono recording

The refcap capture device supports mono recording only, this patch
adds the channel constraints.

Signed-off-by: Yong Zhi <yong.zhi@intel.com>
Reviewed-by: Pierre-Louis Bossart <pierre-louis.bossart@linux.intel.com>
Signed-off-by: Mark Brown <broonie@kernel.org>