sound: oxygen: allocate model_data dynamically

Allocate the model-specific data dynamically instead of including it in
the memory block of the card structure.  This will allow us to determine
the actual model after the card creation.

Signed-off-by: Clemens Ladisch <clemens@ladisch.de>
Signed-off-by: Takashi Iwai <tiwai@suse.de>