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

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

Fixes: 51f3baad ("can: mcba_usb: Add support for Microchip CAN BUS Analyzer")
Cc: stable <stable@vger.kernel.org>     # 4.12
Cc: Remigiusz Kołłątaj <remigiusz.kollataj@mobica.com>
Reported-by:  <syzbot+e29b17e5042bbc56fae9@syzkaller.appspotmail.com>
Signed-off-by: Johan Hovold <johan@kernel.org>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>