ASoC: SOF: nocodec: conditionally set dpcm_capture/dpcm_playback flags

With additional checks on dailinks, we see errors such as

[ 3.000418] sof-nocodec sof-nocodec: CPU DAI DMIC01 Pin for rtd
NoCodec-6 does not support playback

It's not clear why we set the dpcm_playback and dpcm_capture flags
unconditionally, add a check on number of channels for each direction
to avoid invalid configurations.

Fixes: 8017b8fd

 ('ASoC: SOF: Add Nocodec machine driver support')
Signed-off-by: Pierre-Louis Bossart <pierre-louis.bossart@linux.intel.com>
Reviewed-by: Guennadi Liakhovetski <guennadi.liakhovetski@linux.intel.com>
Reviewed-by: Daniel Baluta <daniel.baluta@gmail.com>
Reviewed-by: Bard Liao <yung-chuan.liao@linux.intel.com>
Link: https://lore.kernel.org/r/20200608194415.4663-5-pierre-louis.bossart@linux.intel.com
Signed-off-by: Mark Brown <broonie@kernel.org>