net: stmmac: dwmac-mediatek: disable rx watchdog

disable rx watchdog for dwmac-mediatek, then the hw will
issue a rx interrupt once receiving a packet, so the responding time
for rx path will be reduced.

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