[ethernet] Use standard 1500 byte MTU unless explicitly overridden

Devices that support jumbo frames will currently default to the
largest possible MTU.  This assumption is valid for virtual adapters
such as virtio-net, where the MTU must have been configured by a
system administrator, but is unsafe in the general case of a physical
adapter.

Default to the standard Ethernet MTU, unless explicitly overridden
either by the driver or via the ${netX/mtu} setting.

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/drivers/net/ena.c

 	feature = &rsp->get_feature.feature;
 	memcpy ( netdev->hw_addr, feature->device.mac, ETH_ALEN );
 	netdev->max_pkt_len = le32_to_cpu ( feature->device.mtu );
+	netdev->mtu = ( netdev->max_pkt_len - ETH_HLEN );
 
 	DBGC ( ena, "ENA %p MAC %s MTU %zd\n",
 	       ena, eth_ntoa ( netdev->hw_addr ), netdev->max_pkt_len );

src/drivers/net/virtio-net.c

 			 &mtu, sizeof ( mtu ) );
 		DBGC ( virtnet, "VIRTIO-NET %p mtu=%d\n", virtnet, mtu );
 		netdev->max_pkt_len = ( mtu + ETH_HLEN );
+		netdev->mtu = mtu;
 	}
 
 	/* Register network device */

src/net/ethernet.c

 		netdev->ll_protocol = &ethernet_protocol;
 		netdev->ll_broadcast = eth_broadcast;
 		netdev->max_pkt_len = ETH_FRAME_LEN;
+		netdev->mtu = ETH_MAX_MTU;
 	}
 	return netdev;
 }