arcnet: add netif_carrier_on/off for reconnect

The arcnet device has no interrupt to detect if the link has changed
from disconnected to connected. This patch adds an timer to toggle the
link detection. The timer will get retriggered as long as the
reconnection interrupts accure. If the recon interrupts hold off
for >1s we define the connection stable again.

Signed-off-by: Michael Grzeschik <m.grzeschik@pengutronix.de>