net: mdio: Check regmap pointer returned by device_node_to_regmap()

In the Linux kernel, the following vulnerability has been resolved: net: mdio: Check regmap pointer returned by device_node_to_regmap() The call to device_node_to_regmap() in airoha_mdio_probe() can return an ERR_PTR() if regmap initialization fails. Currently, the driver stores the pointer without validation, which could lead to a crash if it is later dereferenced. Add an IS_ERR() check and return the corresponding error code to make the probe path more robust.