drivers: ethernet: phy_mii: Return -ENOTSUP for fixed-link config

Return -ENOTSUP in phy_mii_cfg_link when a fixed-link configuration is set,
indicating that MDIO read/write operations are not supported.

Signed-off-by: Ofir Shemesh <ofirshemesh777@gmail.com>

Author: ofirshe

drivers/ethernet/phy/phy_mii.c

@@ -315,10 +315,15 @@ static int phy_mii_cfg_link(const struct device *dev,
 			    enum phy_link_speed adv_speeds)
 {
 	struct phy_mii_dev_data *const data = dev->data;
+	const struct phy_mii_dev_config *const cfg = dev->config;
 	uint16_t anar_reg;
 	uint16_t bmcr_reg;
 	uint16_t c1kt_reg;
 
+	if (cfg->fixed) {
+		return -ENOTSUP;
+	}
+
 	if (phy_mii_reg_read(dev, MII_ANAR, &anar_reg) < 0) {
 		return -EIO;
 	}