net: mtk_eth_soc: correct 802.3z speed setting

Phylink does not guarantee that state->speed will be set correctly in
the mac_config() call, so it's a bug that the driver makes use of it.
Moreover, it is making use of it in a function that is only ever called
for 1000BASE-X and 2500BASE-X which operate at a fixed speed which
happens to be the same setting irrespective of the interface mode. We
can simply remove the switch statement and just set the SGMII interface
speed.

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