net: mediatek: mtk_eth_soc: clear MAC_MCR_FORCE_LINK only when MAC is up

Clearing bit MAC_MCR_FORCE_LINK which forces the link down too early
can result in MAC ending up in a broken/blocked state.

Fix this by handling this bit in the .mac_link_up and .mac_link_down
calls instead of in .mac_finish.

Fixes: b8fc9f30 ("net: ethernet: mediatek: Add basic PHYLINK support")
Suggested-by: Mason-cw Chang <Mason-cw.Chang@mediatek.com>
Signed-off-by: Daniel Golle <daniel@makrotopia.org>
Signed-off-by: David S. Miller <davem@davemloft.net>