ALSA: oxygen: Xonar DG(X): make model_xonar_dg const

Make this const as it not modified anywhere. It is only used during a
copy operation. Also, add const to the declaration in header.

Signed-off-by: Bhumika Goyal <bhumirks@gmail.com>
Acked-by: Clemens Ladisch <clemens@ladisch.de>
Signed-off-by: Takashi Iwai <tiwai@suse.de>