ieee802154: atusb: fix use-after-free at disconnect

The disconnect callback was accessing the hardware-descriptor private
data after having having freed it.

Fixes: 7490b008 ("ieee802154: add support for atusb transceiver")
Cc: stable <stable@vger.kernel.org>     # 4.2
Cc: Alexander Aring <alex.aring@gmail.com>
Reported-by:  <syzbot+f4509a9138a1472e7e80@syzkaller.appspotmail.com>
Signed-off-by: Johan Hovold <johan@kernel.org>
Signed-off-by: Stefan Schmidt <stefan@datenfreihafen.org>