batman-adv: use ETH_HLEN instead of sizeof(struct ethhdr)

Instead of using sizeof(struct ethhdr) it is strongly recommended to use the
kernel macro ETH_HLEN. This patch substitute each occurrence of the former
expressione with the latter one.

Signed-off-by: Antonio Quartulli <ordex@autistici.org>
diff --git a/net/batman-adv/soft-interface.c b/net/batman-adv/soft-interface.c
index efe0fba..6e2530b 100644
--- a/net/batman-adv/soft-interface.c
+++ b/net/batman-adv/soft-interface.c
@@ -292,7 +292,7 @@
 /*	skb->ip_summed = CHECKSUM_UNNECESSARY;*/
 
 	bat_priv->stats.rx_packets++;
-	bat_priv->stats.rx_bytes += skb->len + sizeof(struct ethhdr);
+	bat_priv->stats.rx_bytes += skb->len + ETH_HLEN;
 
 	soft_iface->last_rx = jiffies;