realtek: 6.12: harden the mdio bus even more

realtek: 6.12: harden the mdio bus even more

With the follow up EEE patches the mdio bus will run c22 and c45
accesses during initial scan. Especially when accessing addresses
beyond the CPU port phy requests might fail in a way that cannot
be handled gratefully. Do two things

- do not allow access to addresses starting from cpu port
- set the scan disable bitmask to ports starting from cpu port

Signed-off-by: Markus Stockhausen <markus.stockhausen@gmx.de>
Link: https://github.com/openwrt/openwrt/pull/18935
Signed-off-by: Robert Marko <robimarko@gmail.com>