iio:potentiostat:lmp91000: invert if statement

Invert if statement arms in line 214, in order to make the code cleaner,
solve these checkpatch.pl CHECKs:

- lmp9100.c:214: CHECK: braces {} should be used on all arms of this statement
- lmp9100.c:216: CHECK: Unbalanced braces around else statement
Signed-off-by: Lucas Oshiro <lucasseikioshiro@gmail.com>
Signed-off-by: Anderson Reis <andersonreisrosa@gmail.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

drivers/iio/potentiostat/lmp91000.c

@@ -211,12 +211,11 @@ static int lmp91000_read_config(struct lmp91000_data *data)
 
 	ret = of_property_read_u32(np, "ti,tia-gain-ohm", &val);
 	if (ret) {
-		if (of_property_read_bool(np, "ti,external-tia-resistor"))
-			val = 0;
-		else {
+		if (!of_property_read_bool(np, "ti,external-tia-resistor")) {
 			dev_err(dev, "no ti,tia-gain-ohm defined");
 			return ret;
 		}
+		val = 0;
 	}
 
 	ret = -EINVAL;