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: 5e1a1da0f8c9 ("iio: adc: at91-sama5d2_adc: add hw trigger and buffer support")
Signed-off-by: Dan Carpenter <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Jonathan Cameron <[e-mail]>

drivers/iio/adc/at91-sama5d2_adc.c

···
 	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);