ALSA: usb-audio: make read-only array marker static const

Don't populate the read-only array marker on the stack but instead make
it static const. Also makes the object code a little smaller.

Signed-off-by: Colin Ian King <colin.i.king@gmail.com>
Link: https://lore.kernel.org/r/20220809181544.3046429-1-colin.i.king@gmail.com


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