iio:dac:ad3552r: Fix an IS_ERR() vs NULL check

The fwnode_get_named_child_node() function does not return error
pointers.  It returns NULL.  Update the check accordingly.

Fixes: 8f2b5482 ("drivers:iio:dac: Add AD3552R driver support")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Nuno Sá <nuno.sa@analog.com>
Link: https://lore.kernel.org/r/20220404114244.GA19201@kiliSigned-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

drivers/iio/dac/ad3552r.c

@@ -809,10 +809,10 @@ static int ad3552r_configure_custom_gain(struct ad3552r_desc *dac,
 
 	gain_child = fwnode_get_named_child_node(child,
 						 "custom-output-range-config");
-	if (IS_ERR(gain_child)) {
+	if (!gain_child) {
 		dev_err(dev,
 			"mandatory custom-output-range-config property missing\n");
-		return PTR_ERR(gain_child);
+		return -EINVAL;
 	}
 
 	dac->ch_data[ch].range_override = 1;