net: mtk_eth_soc: use resolved link config in mac_link_up()

Convert the mtk_eth_soc driver to use the finalised link parameters in
mac_link_up() rather than the parameters in mac_config().

Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>