net: amd-xgbe: use EOPNOTSUPP instead of ENOTSUPP in xgbe_phy_mii_read_c45

The MDIO read callback xgbe_phy_mii_read_c45() can propagate its return
value up through phylink_mii_ioctl() to user space via netdev ioctls such
as SIOCGMIIREG. Returning ENOTSUPP results in user space seeing
"Unknown error", since ENOTSUPP is not a standard errno value.

Replace ENOTSUPP with EOPNOTSUPP to align with the MDIO core’s
usage and ensure user space receives a proper "Operation not supported"
error instead of an unknown code.

Signed-off-by: Alok Tiwari <[e-mail]>
Reviewed-by: Simon Horman <[e-mail]>
Acked-by: Shyam Sundar S K <[e-mail]>
Link: https://patch.msgid.link/[e-mail]
Signed-off-by: Jakub Kicinski <[e-mail]>

drivers/net/ethernet/amd/xgbe/xgbe-phy-v2.c

···
 	else if (phy_data->conn_type & XGBE_CONN_TYPE_MDIO)
 		ret = xgbe_phy_mdio_mii_read_c45(pdata, addr, devad, reg);
 	else
-		ret = -ENOTSUPP;
+		ret = -EOPNOTSUPP;
 
 	xgbe_phy_put_comm_ownership(pdata);