net: ethernet: mediatek: fix fails from TX housekeeping due to incorrect port setup

which net device the SKB is complete for depends on the forward port
on txd4 on the corresponding TX descriptor, but the information isn't
set up well in case of  SKB fragments that would lead to watchdog timeout
from the upper layer, so fix it up.

Signed-off-by: Sean Wang <sean.wang@mediatek.com>
Acked-by: John Crispin <john@phrozen.org>
Signed-off-by: David S. Miller <davem@davemloft.net>