ALSA: usb: Constify snd_device_ops definitions

Now we may declare const for snd_device_ops definitions, so let's do
it for optimization.

There should be no functional changes by this patch.

Link: https://lore.kernel.org/r/20200103081714.9560-11-tiwai@suse.de


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