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>

sound/soc/intel/skylake/skl.c

@@ -438,7 +438,6 @@ static int skl_free(struct hdac_bus *bus)
 
 	snd_hdac_ext_bus_exit(bus);
 
-	cancel_work_sync(&skl->probe_work);
 	if (IS_ENABLED(CONFIG_SND_SOC_HDAC_HDMI)) {
 		snd_hdac_display_power(bus, HDA_CODEC_IDX_CONTROLLER, false);
 		snd_hdac_i915_exit(bus);
@@ -867,6 +866,9 @@ static int skl_create(struct pci_dev *pci,
 	hbus = skl_to_hbus(skl);
 	bus = skl_to_bus(skl);
 
+	INIT_LIST_HEAD(&skl->ppl_list);
+	INIT_LIST_HEAD(&skl->bind_list);
+
 #if IS_ENABLED(CONFIG_SND_SOC_INTEL_SKYLAKE_HDAUDIO_CODEC)
 	ext_ops = snd_soc_hdac_hda_get_ops();
 #endif
@@ -1116,6 +1118,7 @@ static void skl_remove(struct pci_dev *pci)
 	struct hdac_bus *bus = pci_get_drvdata(pci);
 	struct skl *skl = bus_to_skl(bus);
 
+	cancel_work_sync(&skl->probe_work);
 	release_firmware(skl->tplg);
 
 	pm_runtime_get_noresume(&pci->dev);