iio: adc: mcp3911: return proper error code on failure to allocate trigger

smatch warnings:
drivers/iio/adc/mcp3911.c:441 mcp3911_probe() warn: passing zero to 'PTR_ERR'

Fixes: 08a65f61 ("iio: adc: mcp3911: add support for interrupts")
Reported-by: Nkernel test robot <lkp@intel.com>
Reported-by: NDan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: NMarcus Folkesson <marcus.folkesson@gmail.com>
Link: https://lore.kernel.org/r/20220927092537.94663-1-marcus.folkesson@gmail.comSigned-off-by: NJonathan Cameron <Jonathan.Cameron@huawei.com>

drivers/iio/adc/mcp3911.c

@@ -496,7 +496,7 @@ static int mcp3911_probe(struct spi_device *spi)
 				indio_dev->name,
 				iio_device_id(indio_dev));
 		if (!adc->trig)
-			return PTR_ERR(adc->trig);
+			return -ENOMEM;
 
 		adc->trig->ops = &mcp3911_trigger_ops;
 		iio_trigger_set_drvdata(adc->trig, adc);