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 <[e-mail]>
Signed-off-by: Nuno Sa <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Jonathan Cameron <[e-mail]>

drivers/iio/adc/ad9467.c

···
 	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)