NET: Do not use dev->hard_header_len in eth_header()

The actual return value of eth_header() is never used, only its sign.
So it does not make a difference if we return dev->hard_header_len or
ETH_HLEN, but the latter makes more sense as that is the number of bytes
we added to the front of the frame.

For 99% of the drivers, dev->hard_header_len == ETH_HLEN, so no difference
at all, but if a driver actually needs additional headroom and thus
has dev->hard_header_len > ETH_HLEN, the process of building the ethernet
header should not care at all. 

net/ethernet/eth.c

@@ -103,16 +103,16 @@ int eth_header(struct sk_buff *skb, struct net_device *dev, unsigned short type,
 	if (dev->flags & (IFF_LOOPBACK|IFF_NOARP)) 
 	{
 		memset(eth->h_dest, 0, dev->addr_len);
-		return(dev->hard_header_len);
+		return ETH_HLEN;
 	}
 	
 	if(daddr)
 	{
 		memcpy(eth->h_dest,daddr,dev->addr_len);
-		return dev->hard_header_len;
+		return ETH_HLEN;
 	}
 	
-	return -dev->hard_header_len;
+	return -ETH_HLEN;
 }