ALSA: usx2y: More constifications

Apply const prefix to each possible place: the string array and the
parameter tables and callers.

Just for minor optimization and no functional changes.

Link: https://lore.kernel.org/r/20200105144823.29547-23-tiwai@suse.de


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