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>

target/linux/realtek/files-6.12/drivers/net/pcs/pcs-rtl-otto.c

@@ -3148,6 +3148,13 @@ static int rtpcs_931x_sds_set_ip_mode(struct rtpcs_serdes *sds,
 		mode_val = 0x9;
 		break;
 
+	case RTPCS_SDS_MODE_2500BASEX:
+		/* available SDK code doesn't have this value. based on brute-forcing
+		 * the SerDes mode register field until the link is working
+		 */
+		mode_val = 0x2d;
+		break;
+
 	case RTPCS_SDS_MODE_10GBASER:
 		mode_val = 0x35;
 		break;