staging: iio: ad799x: remove some unneeded IS_ERR() checks

My static checker is upset that we check IS_ERR(t->reg) when we know it
is not an ERR_PTR.

Checking for IS_ERR() twice is often a sign of confusion and buggy code.
In this case, if the call to "ret = regulator_enable(st->vref);" fails,
then we call "regulator_disable(st->vref);" and that's a mistake because
"st->vref" is not enabled.

I fixed these problems and Hartmut Knaack pointed out a couple unneeded
IS_ERR() checks in ad799x_remove() so I have removed those as well.
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Acked-by: Hartmut Knaack <knaack.h@gmx.de>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>

drivers/iio/adc/ad799x.c

@@ -717,7 +717,7 @@ static int ad799x_probe(struct i2c_client *client,
 	ret = iio_triggered_buffer_setup(indio_dev, NULL,
 		&ad799x_trigger_handler, NULL);
 	if (ret)
-		goto error_disable_reg;
+		goto error_disable_vref;
 
 	if (client->irq > 0) {
 		ret = devm_request_threaded_irq(&client->dev,
@@ -739,10 +739,9 @@ static int ad799x_probe(struct i2c_client *client,
 
 error_cleanup_ring:
 	iio_triggered_buffer_cleanup(indio_dev);
-error_disable_reg:
-	if (!IS_ERR(st->vref))
+error_disable_vref:
 	regulator_disable(st->vref);
-	if (!IS_ERR(st->reg))
+error_disable_reg:
 	regulator_disable(st->reg);
 
 	return ret;
@@ -756,9 +755,7 @@ static int ad799x_remove(struct i2c_client *client)
 	iio_device_unregister(indio_dev);
 
 	iio_triggered_buffer_cleanup(indio_dev);
-	if (!IS_ERR(st->vref))
 	regulator_disable(st->vref);
-	if (!IS_ERR(st->reg))
 	regulator_disable(st->reg);
 	kfree(st->rx_buf);