ALSA: usb-audio: More constifications

commit a01df925d1bbc97d6f7fe07b157aadb565315337 upstream.

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>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>