net: ethernet: mtk_eth_soc: remove mac_pcs_get_state and modernise

Remove the .mac_pcs_get_state function, since as far as I can tell is
never called - no DT appears to specify an in-band-status management
nor SFP support for this driver.

Removal of this, along with the previous patch to remove the incorrect
clocking configuration, means that the driver becomes non-legacy, so
we can remove the "legacy_pre_march2020" status from this driver.

Reviewed-by: Daniel Golle <daniel@makrotopia.org>
Tested-by: Daniel Golle <daniel@makrotopia.org>
Tested-by: Frank Wunderlich <frank-w@public-files.de>
Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>