iio: adc: ad7606: check for NULL before calling sw_mode_config()

Check that the sw_mode_config function pointer is not NULL before
calling it. Not all buses define this callback, which resulted in a NULL
pointer dereference.

Fixes: e571c19 ("iio: adc: ad7606: move scale_setup as function pointer on chip-info")
Reviewed-by: Nuno Sá <nuno.sa@analog.com>
Signed-off-by: David Lechner <dlechner@baylibre.com>
Link: https://patch.msgid.link/20250318-iio-adc-ad7606-improvements-v2-1-4b605427774c@baylibre.com
Cc: <Stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

Author: dlech

drivers/iio/adc/ad7606.c

@@ -1236,9 +1236,11 @@ static int ad7616_sw_mode_setup(struct iio_dev *indio_dev)
 	st->write_scale = ad7616_write_scale_sw;
 	st->write_os = &ad7616_write_os_sw;
 
-	ret = st->bops->sw_mode_config(indio_dev);
-	if (ret)
-		return ret;
+	if (st->bops->sw_mode_config) {
+		ret = st->bops->sw_mode_config(indio_dev);
+		if (ret)
+			return ret;
+	}
 
 	/* Activate Burst mode and SEQEN MODE */
 	return ad7606_write_mask(st, AD7616_CONFIGURATION_REGISTER,
@@ -1268,6 +1270,9 @@ static int ad7606b_sw_mode_setup(struct iio_dev *indio_dev)
 	st->write_scale = ad7606_write_scale_sw;
 	st->write_os = &ad7606_write_os_sw;
 
+	if (!st->bops->sw_mode_config)
+		return 0;
+
 	return st->bops->sw_mode_config(indio_dev);
 }