ALSA: vx: More constifications

Apply const prefix to every possible place: the static tables for DSP
commands, the string tables, and register/offset tables.

Just for minor optimization and no functional changes.

Link: https://lore.kernel.org/r/20200105144823.29547-8-tiwai@suse.de


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