ALSA: ac97: Treat snd_ac97_bus_ops as const

This is a preliminary patch to allow const for snd_ac97_bus_ops
definitions in each driver's code.  The ops reference is read-only,
hence it can be declared as const for further optimization.

There should be no functional changes by this patch.

Link: https://lore.kernel.org/r/20200103081714.9560-23-tiwai@suse.de


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