net: revert "net: l2tp: Treat NET_XMIT_CN as success in l2tp_eth_dev_xmit"

This reverts commit 7c6ae610

, because l2tp_xmit_skb() never
returns NET_XMIT_CN, it ignores the return value of l2tp_xmit_core().

Cc: Gao Feng <gfree.wind@gmail.com>
Signed-off-by: Cong Wang <xiyou.wangcong@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>