net: lantiq_etop: don't free skb when returning NETDEV_TX_BUSY

The ndo_start_xmit() method must not free skb when returning
NETDEV_TX_BUSY, since caller is going to requeue freed skb.

Fixes: 504d4721 ("MIPS: Lantiq: Add ethernet driver")
Signed-off-by: Zhang Changzhong <zhangchangzhong@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>