ALSA: ac97: Constify snd_ac97_res_table definition

One snd_ac97_res_table definition remains forgotten without const.
Let's add it for a bit of optimization.

There should be no functional changes by this patch.

Link: https://lore.kernel.org/r/20200103081714.9560-46-tiwai@suse.de


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