can: kvaser_usb: Use can-dev unregistration mechanism

Use can-dev's unregister_candev() instead of directly calling
networking unregister_netdev(). While both are functionally
equivalent, unregister_candev() might do extra stuff in the
future than just calling networking layer unregistration code.

Signed-off-by: Ahmed S. Darwish <ahmed.darwish@valeo.com>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>