ALSA: oxfw: enable to keep memory block for model-specific structure

ALSA oxfw driver should have backward compatibility to old
firewire-speakers driver. Additionally, in future commit, scs1x driver
will be merged. It's nice to add a pointer to have a memory block for
model-specific structures.

This commit adds a member to 'struct snd_oxfw' for this aim. Deallocation
is done at freeing ALSA card structure.

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