r6040: use ETH_ZLEN instead of MISR for SKB length checking

Ever since this driver was merged the following code was included:

if (skb->len < MISR)
	skb->len = MISR;

MISR is defined to 0x3C which is also equivalent to ETH_ZLEN, but use
ETH_ZLEN directly which is exactly what we want to be checking for.
Reported-by: Marc Volovic <marcv@ezchip.com>
Signed-off-by: Florian Fainelli <florian@openwrt.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/ethernet/rdc/r6040.c

@@ -835,8 +835,8 @@ static netdev_tx_t r6040_start_xmit(struct sk_buff *skb,
 	/* Set TX descriptor & Transmit it */
 	lp->tx_free_desc--;
 	descptr = lp->tx_insert_ptr;
-	if (skb->len < MISR)
-		descptr->len = MISR;
+	if (skb->len < ETH_ZLEN)
+		descptr->len = ETH_ZLEN;
 	else
 		descptr->len = skb->len;