can: usb_8dev: unregister netdev before free()ing

The usb_8dev hardware has problems on some xhci USB hosts. The driver fails to
read the firmware revision in the probe function. This leads to the following
Oops:

    [ 3356.635912] kernel BUG at net/core/dev.c:5701!

The driver tries to free the netdev, which has already been registered, without
unregistering it.

This patch fixes the problem by unregistering the netdev in the error path.

Reported-by: Michael Olbrich <m.olbrich@pengutronix.de>
Reviewed-by: Bernd Krumboeck <krumboeck@universalnet.at>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>