USB: audio: guard kernel-only code with __KERNEL__

include/linux/usb/audio.h is exported to userspace,
so part of this file that is for internal kernel
usage need to be guarded with ifdef __KERNEL__.
This way make headers_install will stript it out.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>