ASoC: Intel: Skylake: simplify boolean tests

Detected with Coccinelle

skl-messages.c:419:5-32: WARNING: Comparison to bool
skl-pcm.c:1426:6-33: WARNING: Comparison to bool

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