ASoC: Intel: Skylake: Make local functions static

Some functions are local to the source and do not need to be in global
scope, so make them static.

Signed-off-by: Guneshwor Singh <guneshwor.o.singh@intel.com>
Acked-By: Vinod Koul <vinod.koul@intel.com>
Signed-off-by: Mark Brown <broonie@kernel.org>