ASoC: dpcm: don't merge format from invalid codec dai

When merging codec formats, dpcm_runtime_base_format() should skip
the codecs which are not supporting the current stream direction.

At the moment, if a BE link has more than one codec, and only one
of these codecs has no capture DAI, it becomes impossible to start
a capture stream because the merged format would be 0.

Skipping invalid codec DAI solves the problem.

Fixes: b073ed4e ("ASoC: soc-pcm: DPCM cares BE format")
Signed-off-by: Jerome Brunet <jbrunet@baylibre.com>
Signed-off-by: Mark Brown <broonie@kernel.org>
Cc: stable@vger.kernel.org