ALSA: ac97: Constify static struct attribute_group

The only usage of ac97_adapter_attr_group is to put its address in an
array of pointers to const attribute_group structs. Make it const to
allow the compiler to put it in read-only memory.

Signed-off-by: Rikard Falkeborn <rikard.falkeborn@gmail.com>
Link: https://lore.kernel.org/r/20210131001241.2278-2-rikard.falkeborn@gmail.com
Signed-off-by: Takashi Iwai <tiwai@suse.de>