ARCNET: return IRQ_NONE if the interface isn't running

The interrupt handler needs to return IRQ_NONE in case
two devices are used with the shared interrupt handler.
Otherwise it could steal interrupts from the other
interface.

Signed-off-by: Michael Grzeschik <m.grzeschik@pengutronix.de>
Signed-off-by: David S. Miller <davem@davemloft.net>