ALSA: ac97c: constify ac97_pcm structures

ac97_pcm are not supposed to change at runtime. All functions
working with ac97_pcm provided by <sound/ac97_codec.h> work with
const ac97_pcm. 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>