net: mtk_eth_soc: add fixme comment for state->speed use

Add a fixme comment for the last remaining incorrect usage of
state->speed in the mac_config() method, which is strangely in a code
path which is only run when the PHY interface mode changes.

This means if we are in RGMII mode, changes in state->speed will not
cause the INTF_MODE, TRGMII_RCK_CTRL and TRGMII_TCK_CTRL registers to
be set according to the speed, nor will the TRGPLL clock be set to the
correct value.

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