The snd_pcm_ops items defined in snd-dummy driver can be gracefully
declared as const.  Let's mark them for further optimization.

There should be no functional changes by this patch.

Link: https://lore.kernel.org/r/20200103081714.9560-44-tiwai@suse.deSigned-off-by: Takashi Iwai <tiwai@suse.de>