ARCNET: fix hard_header_len limit

For arcnet the bare minimum header only contains the 4 bytes to
specify source, dest and offset (1, 1 and 2 bytes respectively).
The corresponding struct is struct arc_hardware.

The struct archdr contains additionally a union of possible soft
headers. When doing $insertusecasehere packets might well
include short (or even no?) soft headers.

For this reason only use arc_hardware instead of archdr to
determine the hard_header_len for an arcnet device.
Signed-off-by: NMichael Grzeschik <m.grzeschik@pengutronix.de>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

drivers/net/arcnet/arcnet.c

@@ -326,7 +326,7 @@ static void arcdev_setup(struct net_device *dev)
 	dev->type = ARPHRD_ARCNET;
 	dev->netdev_ops = &arcnet_netdev_ops;
 	dev->header_ops = &arcnet_header_ops;
-	dev->hard_header_len = sizeof(struct archdr);
+	dev->hard_header_len = sizeof(struct arc_hardware);
 	dev->mtu = choose_mtu();
 
 	dev->addr_len = ARCNET_ALEN;