iio: adc: ad9467: use the more common !val NULL check

Check !val instead of directing checking for NULL (val == NULL).
No functional changes intended.
Reviewed-by: David Lechner <dlechner@baylibre.com>
Signed-off-by: Nuno Sa <nuno.sa@analog.com>
Link: https://lore.kernel.org/r/20231207-iio-backend-prep-v2-7-a4a33bc4d70e@analog.comSigned-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

drivers/iio/adc/ad9467.c

@@ -158,7 +158,7 @@ static int ad9467_reg_access(struct adi_axi_adc_conv *conv, unsigned int reg,
 	struct spi_device *spi = st->spi;
 	int ret;
 
-	if (readval == NULL) {
+	if (!readval) {
 		guard(mutex)(&st->lock);
 		ret = ad9467_spi_write(spi, reg, writeval);
 		if (ret)