net: mtk_eth_soc: correct 802.3z duplex setting

Phylink does not guarantee that state->duplex will be set correctly in
the mac_config() call, so it's a bug that the driver makes use of it.

Move the 802.3z PCS duplex configuration to mac_link_up().

Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>