net: lantiq_etop: Fix return type for implementation of ndo_start_xmit

Since Linux now supports CFI, it will be a good idea to fix mismatched
return type for implementation of hooks. Otherwise this might get
cought out by CFI and cause a panic.

ltq_etop_tx() would return either NETDEV_TX_BUSY or NETDEV_TX_OK, so
change the return type to netdev_tx_t directly.

Signed-off-by: GUO Zihua <guozihua@huawei.com>
Link: https://lore.kernel.org/r/20220902081521.59867-1-guozihua@huawei.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>