net: ethernet: mtk-star-emac: enable half duplex hardware support

Current driver doesn't support half duplex correctly.
This patch enable half duplex capability in hardware.

Signed-off-by: Biao Huang <biao.huang@mediatek.com>
Signed-off-by: Yinghua Pan <ot_yinghua.pan@mediatek.com>
Signed-off-by: David S. Miller <davem@davemloft.net>