ASoC: (cosmetic) simplify dpcm_prune_paths()

ASoC: (cosmetic) simplify dpcm_prune_paths()

Currently dpcm_prune_paths() has up to 4 nested condition and loop
levels, which forces the code to use flags for flow control.
Extracting widget status verification code from dpcm_prune_paths()
into a separate function simplifies the code.

Signed-off-by: Guennadi Liakhovetski <guennadi.liakhovetski@linux.intel.com>
Link: https://lore.kernel.org/r/20200312095214.15126-2-guennadi.liakhovetski@linux.intel.com
Signed-off-by: Mark Brown <broonie@kernel.org>