ASoC: soc-pcm: skip dpcm path checking with incapable/unready FE

Skip dpcm path checking for playback or capture, if corresponding FE
doesn't support playback or capture, or currently is not ready. It
can reduce the unnecessary cost to search connected widgets.

[Tweaked comments for clarity -- broonie]

Signed-off-by: Qiao Zhou <zhouqiao@marvell.com>
Signed-off-by: Mark Brown <broonie@kernel.org>