realtek: make use of serdes helper for Zyxel GS1900-24(HP) v1/v2

realtek: make use of serdes helper for Zyxel GS1900-24(HP) v1/v2

Use the new INTERNAL_PHY_SDS() helper to describe the SFP ports. With
this change the driver now knows that ports 24/26 are driven by serdes
4/5.

For the RTL838x devices this is currently only an additional information
for the mdio bus. It is not evaluated further because everything is
hardcoded.

Signed-off-by: Markus Stockhausen <markus.stockhausen@gmx.de>
Link: https://github.com/openwrt/openwrt/pull/18851
Signed-off-by: Robert Marko <robimarko@gmail.com>