USB: Move vendor subclass definition from usb/audio.h to usb/ch9.h

USB_SUBCLASS_VENDOR_SPEC is common to several USB classes and as such belongs
to usb/ch9.h.

Signed-off-by: Laurent Pinchart <laurent.pinchart@skynet.be>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>