ALSA: au88x0: More constifications

Apply const prefix to each possible place: the static register tables,
the coef tables, the string arrays, etc.

Just for minor optimization and no functional changes.

Link: https://lore.kernel.org/r/20200105144823.29547-14-tiwai@suse.de


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