net: ethernet: mediatek: fix inconsistency of port number carried in TXD

Fix port inconsistency on TXD due to hardware BUG that would cause
different port number is carried on the same TXD between tx_map()
and tx_unmap() with the iperf test. It would cause confusing BQL
logic which leads to kernel panic when dual GMAC runs concurrently.

Signed-off-by: Sean Wang <sean.wang@mediatek.com>
Signed-off-by: David S. Miller <davem@davemloft.net>