Both snd_vx_hardware and snd_vx_ops are only referred without
modification, hence they can be constified gracefully for further
optimizations.

There should be no functional changes by this patch.

Link: https://lore.kernel.org/r/20200103081714.9560-31-tiwai@suse.deSigned-off-by: Takashi Iwai <tiwai@suse.de>