net: allwinner: Fix use correct return type for ndo_start_xmit()

[ Upstream commit 09f6c44aaae0f1bdb8b983d7762676d5018c53bc ]

The method ndo_start_xmit() returns a value of type netdev_tx_t. Fix
the ndo function to use the correct type. And emac_start_xmit() can
leak one skb if 'channel' == 3.

Signed-off-by: Yunjian Wang <wangyunjian@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Sasha Levin <sashal@kernel.org>