iio:adc:ad7923: Rewrite comparison to NULL

Solves checkpath.pl's message:

CHECK: Comparison to NULL could be written "!indio_dev"
Signed-off-by: Bárbara Fernandes <barbara.fernandes@usp.br>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

drivers/iio/adc/ad7923.c

@@ -272,7 +272,7 @@ static int ad7923_probe(struct spi_device *spi)
 	int ret;
 
 	indio_dev = devm_iio_device_alloc(&spi->dev, sizeof(*st));
-	if (indio_dev == NULL)
+	if (!indio_dev)
 		return -ENOMEM;
 
 	st = iio_priv(indio_dev);