ALSA: aoa: More constifications

Apply const prefix at every place where appropriate: the static
register tables, the volume tables and the callback tables.

Just for minor optimization and no functional changes.

Link: https://lore.kernel.org/r/20200105144823.29547-3-tiwai@suse.de


Signed-off-by: Takashi Iwai <tiwai@suse.de>