ALSA: drivers: Constify snd_ac97_bus_ops definitions

Now snd_ac97_bus() takes the const ops pointer, so we can define the
snd_ac97_bus_ops locally as const as well for further optimization.

There should be no functional changes by this patch.

Link: https://lore.kernel.org/r/20200103081714.9560-27-tiwai@suse.de


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