realtek: mdio: rtl838x: activate combo PHY media detection

realtek: mdio: rtl838x: activate combo PHY media detection

There is a misunderstanding about BIT(7) aka EX_PHY_MAN_24_27 in
SMI_GLB_CTRL register. The SDK sets/clears it at different places and
it is not clear what it is for. Observation shows that it is essential
for a working MAC_LINK_MEDIA_STS register.

A RTL838x device has usally two configurations

- port 24/26 are 2 serdes driven fiber ports
- port 24-27 are 4 PHY driven combo ports

In the combo case the above bit must be set so that a switch between
copper and fiber can be detected. Cleanup the MDIO initialization
and remove the unneeded bit handling in the DSA driver.

Signed-off-by: Markus Stockhausen <markus.stockhausen@gmx.de>
Link: https://github.com/openwrt/openwrt/pull/21653
Signed-off-by: Robert Marko <robimarko@gmail.com>