smsc75xx: fix smsc75xx_set_features()

If an attempt is made to disable RX checksums, USB adapter is changed
but netdev->features is not, because smsc75xx_set_features() returns a
non zero value.

This throws errors from netdev_rx_csum_fault() :
<devname>: hw csum failure
Signed-off-by: NEric Dumazet <edumazet@google.com>
Cc: Steve Glendinning <steve.glendinning@shawell.net>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

drivers/net/usb/smsc75xx.c

@@ -954,10 +954,11 @@ static int smsc75xx_set_features(struct net_device *netdev,
 	/* it's racing here! */
 
 	ret = smsc75xx_write_reg(dev, RFE_CTL, pdata->rfe_ctl);
-	if (ret < 0)
+	if (ret < 0) {
 		netdev_warn(dev->net, "Error writing RFE_CTL\n");
-
-	return ret;
+		return ret;
+	}
+	return 0;
 }
 
 static int smsc75xx_wait_ready(struct usbnet *dev, int in_pm)