net: fec: fix improper use of NETDEV_TX_BUSY

net: fec: fix improper use of NETDEV_TX_BUSY

[ Upstream commit 06a4df5863f73af193a4ff7abf7cb04058584f06 ]

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

Fix it by returning NETDEV_TX_OK in case of dma_map_single() fails.

Fixes: 79f339125ea3 ("net: fec: Add software TSO support")
Signed-off-by: Zhang Changzhong <zhangchangzhong@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Sasha Levin <sashal@kernel.org>