realtek: pcs: rtl931x: add 2500Base-X mode

realtek: pcs: rtl931x: add 2500Base-X mode

Add 2500Base-X handling to mode setting which was rejected with
-ENOTSUPP before. SDK code available to us doesn't have the proper mode
value. Though by brute-forcing different mode values, 0x2d was found to
make a 2500Base-X link work.

This was tested with an otherwise correctly configured RTL8221B PHY
which is automatically switched between 2500Base-X and SGMII in the
upstream driver. Though, since there was a previous U-Boot setup for the
PHY in HISGMII mode, it may not be standalone yet.

Signed-off-by: Jonas Jelonek <jelonek.jonas@gmail.com>
Link: https://github.com/openwrt/openwrt/pull/22014
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>