ALSA: nm256: constify snd_ac97_res_table

snd_ac97_res_table are not supposed to change at runtime. All functions
working with snd_ac97_res_table provided by <sound/ac97_codec.h> work with
const snd_ac97_res_table. So mark the non-const structs as const.

Signed-off-by: Arvind Yadav <arvind.yadav.cs@gmail.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>