net: pxa168_eth: remove unused variable 'retval' int pxa168_eth_change_mtu()

Fixes the following W=1 kernel build warning(s):

drivers/net/ethernet/marvell/pxa168_eth.c:1190:6: warning:
 variable 'retval' set but not used [-Wunused-but-set-variable]
 1190 |  int retval;
      |      ^~~~~~

Function pxa168_eth_change_mtu() always return zero, so variable 'retval'
is redundant, just remove it.
Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Zhang Changzhong <zhangchangzhong@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/ethernet/marvell/pxa168_eth.c

@@ -1187,11 +1187,10 @@ static int pxa168_eth_stop(struct net_device *dev)
 
 static int pxa168_eth_change_mtu(struct net_device *dev, int mtu)
 {
-	int retval;
 	struct pxa168_eth_private *pep = netdev_priv(dev);
 
 	dev->mtu = mtu;
-	retval = set_port_config_ext(pep);
+	set_port_config_ext(pep);
 
 	if (!netif_running(dev))
 		return 0;