net: phy: realtek: fix in-band capabilities for 2.5G PHYs

It looks like the configuration of in-band AN only affects SGMII, and it
is always disabled for 2500Base-X. Adjust the reported capabilities
accordingly.

This is based on testing using OpenWrt on Zyxel XGS1010-12 rev A1 with
RTL8226-CG, and Zyxel XGS1210-12 rev B1 with RTL8221B-VB-CG. On these
devices, 2500Base-X in-band AN is known to work with some SFP modules
(containing an unknown PHY). However, with the built-in Realtek PHYs,
no auto-negotiation takes place, irrespective of the configuration of
the PHY.

Fixes: 10fbd71fc5f9b ("net: phy: realtek: implement configuring in-band an")
Signed-off-by: Jan Hoffmann <jan@3e8.eu>
Reviewed-by: Daniel Golle <daniel@makrotopia.org>
Link: https://patch.msgid.link/20260113205557.503409-1-jan@3e8.eu
Signed-off-by: Jakub Kicinski <kuba@kernel.org>