can: kvaser_usb: Ignore spurious error events after a busoff

Sending data in high speed then introducing a busoff results
in spurious BUS_ERROR events from the USBCan-II firmware directly
_after_ the triggered BUS_OFF event.

In the current CAN state handling code, this will lead to an
invalid can state of ACTIVE, ERROR, or PASSIVE even though the
CAN controller has been already shut down due to the busoff.

Guard the state handling code from such invalid events.

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