net: ethernet: mediatek: remove useless code in mtk_poll_tx()

Remove useless local variable _condition_ and the code related.

Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Acked-by: Sean Wang <sean.wang@mediatek.com>
Signed-off-by: David S. Miller <davem@davemloft.net>