ALSA: usx2y: Constify struct snd_usb_audio_quirk entries

The quirk entries used in us122l and usx2y drivers can be declared as
const as they are read-only.

There should be no functional changes by this patch.

Link: https://lore.kernel.org/r/20200103081714.9560-52-tiwai@suse.de


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