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>