ALSA: hda - Sanity check of unexpected cap_sync_hook override

There are a couple of places setting cap_sync_hook in the codec
drivers, and they just overwrite the value.  Add a sanity check via
WARN_ON() in case if an old non-NULL value is overridden and
forgotten.

Signed-off-by: Takashi Iwai <tiwai@suse.de>
diff --git a/sound/pci/hda/patch_realtek.c b/sound/pci/hda/patch_realtek.c
index 32a7a72..d9461ee 100644
--- a/sound/pci/hda/patch_realtek.c
+++ b/sound/pci/hda/patch_realtek.c
@@ -4828,6 +4828,7 @@ static void alc_probe_headset_mode(struct hda_codec *codec)
 			spec->headphone_mic_pin = cfg->inputs[i].pin;
 	}
 
+	WARN_ON(spec->gen.cap_sync_hook);
 	spec->gen.cap_sync_hook = alc_update_headset_mode_hook;
 	spec->gen.automute_hook = alc_update_headset_mode;
 	spec->gen.hp_automute_hook = alc_update_headset_jack_cb;