iio: adc: at91-sama5d2_adc: fix an error code in at91_adc_allocate_trigger()

The at91_adc_allocate_trigger() function is supposed to return error
pointers.  Returning a NULL will cause an Oops.

Fixes: 5e1a1da0 ("iio: adc: at91-sama5d2_adc: add hw trigger and buffer support")
Signed-off-by: Dan Carpenter <error27@gmail.com>
Link: https://lore.kernel.org/r/5d728f9d-31d1-410d-a0b3-df6a63a2c8ba@kili.mountainSigned-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

drivers/iio/adc/at91-sama5d2_adc.c

@@ -1409,7 +1409,7 @@ static struct iio_trigger *at91_adc_allocate_trigger(struct iio_dev *indio,
 	trig = devm_iio_trigger_alloc(&indio->dev, "%s-dev%d-%s", indio->name,
 				iio_device_id(indio), trigger_name);
 	if (!trig)
-		return NULL;
+		return ERR_PTR(-ENOMEM);
 
 	trig->dev.parent = indio->dev.parent;
 	iio_trigger_set_drvdata(trig, indio);