ALSA: pci/au88x0: remove "defined but not used" warnings

Fix W=1 warnings. Mark all unused tables with __maybe_unused.

Signed-off-by: Pierre-Louis Bossart <pierre-louis.bossart@linux.intel.com>
Link: https://lore.kernel.org/r/20200702193604.169059-24-pierre-louis.bossart@linux.intel.com
Signed-off-by: Takashi Iwai <tiwai@suse.de>