sunvnet: fix NULL pointer dereference

This patch fixes a NULL pointer dereference when __tx_port_find() doesn't
find a matching port.

Signed-off-by: David L Stevens <david.stevens@oracle.com>
Acked-by: Sowmini Varadhan <sowmini.varadhan@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>