ASoC: Intel: Skylake: Initialize lists before access so they are safe to use

If skl_probe_work() was not run driver ends up dereferencing NULL
pointer when operating on lists in skl_platform_unregister().
To fix this initialize lists in skl_create(). Also run
cancel_work_sync() before all cleanup functions, so we don't end up
unnecessarily running probe work.

Easily reproducible with:
while true; do modprobe snd_soc_skl; rmmod snd_soc_skl; done
(with the assumption that relevant drivers are added to blacklist on
system boot)

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