um: vector netdev: adjust to xmit_more API change

Replace skb->xmit_more usage by netdev_xmit_more().

Fixes: 4f296ede ("drivers: net: aurora: use netdev_xmit_more helper")
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>