ALSA: oxfw: rename a structure so that it means backward compatibility to old drivers

In former commits, some model-specific members are split from the
structure. The structure is just to keep names for compatibility to old
drivers.

This commit arranges name of the structure and localize it.

Signed-off-by: Takashi Sakamoto <o-takashi@sakamocchi.jp>
Signed-off-by: Takashi Iwai <tiwai@suse.de>