ALSA: usb-audio: More constifications

Apply const prefix to the remaining places: the static table for the
unit information, the mixer maps, the validator tables, etc.

Just for minor optimization and no functional changes.

Link: https://lore.kernel.org/r/20200105144823.29547-12-tiwai@suse.de


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