net: mediatek: setup proper state for disabled GMAC on the default

The current solution would setup fixed and force link of 1Gbps to the both
GMAC on the default. However, The GMAC should always be put to link down
state when the GMAC is disabled on certain target boards. Otherwise,
the driver possibly receives unexpected data from the floating hardware
connection through the unused GMAC. Although the driver had been added
certain protection in RX path to get rid of such kind of unexpected data
sent to the upper stack.

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