realtek: mdio: initialize RTL930x mac type control

realtek: mdio: initialize RTL930x mac type control

For each port (or port group) the mdio bus needs to define the
PHY type that is attached to it. There are the following bit
values that need to be set in SMI_MAC_TYPE_CTRL.

- 0x0: 10G/1G Fiber (SerDes)
- 0x1: 10G/2G5 GPHY
- 0x2: FEPHY
- 0x3: GPHY

SerDes ports are out of scope of the mdio driver and are handled
by the PCS driver. So the corresponding bits are untouched. That
is not good as the register default is 0x3 for ports 0-23. To
make it simple: Without proper setup devices that have SerDes
driven fiber ports at address 0-23 do not poll in the right way.
Link detection is broken.

Fix this by initializing the register to zero. This way all ports
that are not setup by the mdio driver default to "SerDes". That
should be a reasonable assumption.

Fixes: b271735 ("realtek: mdio: Simplify RTL930x phy polling setup")
Reported-by: Joe Holden <jwh@zorins.us>
Suggested-by: Jonas Jelonek <jelonek.jonas@gmail.com>
Signed-off-by: Markus Stockhausen <markus.stockhausen@gmx.de>
Link: https://github.com/openwrt/openwrt/pull/22032
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>