batman-adv: Avoid magic value for minimum MTU

The header linux/if_ether.h already defines a constant for the minimum MTU.
So simply use it instead of having a magic constant in the code.
Signed-off-by: Sven Eckelmann <sven@narfation.org>
Signed-off-by: Simon Wunderlich <sw@simonwunderlich.de>

net/batman-adv/soft-interface.c

@@ -154,7 +154,7 @@ static int batadv_interface_set_mac_addr(struct net_device *dev, void *p)
 static int batadv_interface_change_mtu(struct net_device *dev, int new_mtu)
 {
 	/* check ranges */
-	if (new_mtu < 68 || new_mtu > batadv_hardif_min_mtu(dev))
+	if (new_mtu < ETH_MIN_MTU || new_mtu > batadv_hardif_min_mtu(dev))
 		return -EINVAL;
 
 	dev->mtu = new_mtu;