6lowpan: remove unnecessary check on err >= 0

The err variable can only be zero in this case.
Signed-off-by: Alexander Aring <alex.aring@gmail.com>
Reviewed-by: Werner Almesberger <werner@almesberger.net>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ieee802154/6lowpan.c

@@ -1179,7 +1179,7 @@ lowpan_skb_fragmentation(struct sk_buff *skb, struct net_device *dev)
 	head[0] &= ~LOWPAN_DISPATCH_FRAG1;
 	head[0] |= LOWPAN_DISPATCH_FRAGN;
 
-	while ((payload_length - offset > 0) && (err >= 0)) {
+	while (payload_length - offset > 0) {
 		int len = LOWPAN_FRAG_SIZE;
 
 		head[4] = offset / 8;