[NET]: unregister_netdevice as void

There was no real useful information from the unregister_netdevice() return
code, the only error occurred in a situation that was a driver bug. So
change it to a void function.

Signed-off-by: Stephen Hemminger <shemminger@osdl.org>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/net/ipv4/ipip.c b/net/ipv4/ipip.c
index 9d719d6..da8bbd2 100644
--- a/net/ipv4/ipip.c
+++ b/net/ipv4/ipip.c
@@ -754,7 +754,8 @@
 				goto done;
 			dev = t->dev;
 		}
-		err = unregister_netdevice(dev);
+		unregister_netdevice(dev);
+		err = 0;
 		break;
 
 	default: