iio: dac: mcp4725: fix odd_ptr_err.cocci warnings

PTR_ERR should access the value just tested by IS_ERR

Generated by: scripts/coccinelle/tests/odd_ptr_err.cocci

CC: Tomas Novotny <tomas@novotny.cz>
Signed-off-by: Julia Lawall <julia.lawall@lip6.fr>
Signed-off-by: Fengguang Wu <fengguang.wu@intel.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>

drivers/iio/dac/mcp4725.c

@@ -449,7 +449,7 @@ static int mcp4725_probe(struct i2c_client *client,
 	if (pdata->use_vref) {
 		data->vref_reg = devm_regulator_get(&client->dev, "vref");
 		if (IS_ERR(data->vref_reg)) {
-			err = PTR_ERR(data->vdd_reg);
+			err = PTR_ERR(data->vref_reg);
 			goto err_disable_vdd_reg;
 		}