The method ndo_start_xmit() is defined as returning an 'netdev_tx_t',
which is a typedef for an enum type, also the implementation in this
driver has returns 'netdev_tx_t' value, so just change the function
return type to netdev_tx_t.

Found by coccinelle.
Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>