can: usb_8dev: fix use-after-free on disconnect

The driver was accessing its driver data after having freed it.

Fixes: 0024d8ad ("can: usb_8dev: Add support for USB2CAN interface from 8 devices")
Cc: stable <stable@vger.kernel.org>     # 3.9
Cc: Bernd Krumboeck <b.krumboeck@gmail.com>
Cc: Wolfgang Grandegger <wg@grandegger.com>
Signed-off-by: Johan Hovold <johan@kernel.org>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>