ax88179_178a: fix bonding failure

The following patch fixes a bug which causes the ax88179_178a driver to be
incapable of being added to a bond.

When I brought up the issue with the bonding maintainers, they indicated
that the real problem was with the NIC driver which must return zero for
success (of setting the MAC address). I see that several other NIC drivers
follow that pattern by either simply always returing zero, or by passing
through a negative (error) result while rewriting any positive return code
to zero. With that same philisophy applied to the ax88179_178a driver, it
allows it to work correctly with the bonding driver.

I believe this is suitable for queuing in -stable, as it's a small, simple,
and obvious fix that corrects a defect with no other known workaround.

This patch is against vanilla 3.17(.0).
Signed-off-by: NIan Morgan <imorgan@primordial.ca>

 drivers/net/usb/ax88179_178a.c |    7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

drivers/net/usb/ax88179_178a.c

@@ -937,6 +937,7 @@ static int ax88179_set_mac_addr(struct net_device *net, void *p)
 {
 	struct usbnet *dev = netdev_priv(net);
 	struct sockaddr *addr = p;
+	int ret;
 
 	if (netif_running(net))
 		return -EBUSY;
@@ -946,8 +947,12 @@ static int ax88179_set_mac_addr(struct net_device *net, void *p)
 	memcpy(net->dev_addr, addr->sa_data, ETH_ALEN);
 
 	/* Set the MAC address */
-	return ax88179_write_cmd(dev, AX_ACCESS_MAC, AX_NODE_ID, ETH_ALEN,
+	ret = ax88179_write_cmd(dev, AX_ACCESS_MAC, AX_NODE_ID, ETH_ALEN,
 				 ETH_ALEN, net->dev_addr);
+	if (ret < 0)
+		return ret;
+
+	return 0;
 }
 
 static const struct net_device_ops ax88179_netdev_ops = {