iio: inkern: add out of range error message

If the DT contains an invalid channel specifier then the probe of
iio_hwmon fails with the following message:

iio_hwmon: probe of iio_hwmon failed with error -22

So it's better to print out the relevant channel specifier in
error case to locate the problem.

Signed-off-by: Stefan Wahren <[e-mail]>
Signed-off-by: Jonathan Cameron <[e-mail]>

drivers/iio/inkern.c

···
 	if (!iiospec->args_count)
 		return 0;
 
-	if (iiospec->args[0] >= indio_dev->num_channels)
+	if (iiospec->args[0] >= indio_dev->num_channels) {
+		dev_err(&indio_dev->dev, "invalid channel index %u\n",
+			iiospec->args[0]);
 		return -EINVAL;
+	}
 
 	return iiospec->args[0];
 }