ALSA: oxfw: remove needless member from private structure

In former commit, 'struct device_info' is obsoleted, whereas private
structure still keeps a pointer to it.

This commit remove the member.

d6ce6bbd

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

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