ALSA: snd_usb_caiaq: reparent sound device

The sound device instance needs to be a child of the USB interface, not
the USB device. Newer udev versions pay attention to that.

Signed-off-by: Daniel Mack <daniel@caiaq.de>
Reported-by: Lennart Poettering <lennart@poettering.net>
Signed-off-by: Takashi Iwai <tiwai@suse.de>