ALSA: ad1816a: constify copied structure

The snd_ad1816a_timer_table structure is only copied into
another structure, so make it const.

The opportunity for this change was found using Coccinelle.

Signed-off-by: Julia Lawall <Julia.Lawall@inria.fr>
Link: https://lore.kernel.org/r/1577864614-5543-2-git-send-email-Julia.Lawall@inria.fr


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