iio: mxs-lradc: fix buffer overflow

Fixes:
drivers/staging/iio/adc/mxs-lradc.c:1556 mxs_lradc_probe() error: buffer
overflow 'iio->channels' 15 <= 15

The reported available scales for in_voltage15 were also wrong.

The realbits lookup is not necessary as all the channels of the LRADC have the
same resolution, use LRADC_RESOLUTION instead.
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>

drivers/staging/iio/adc/mxs-lradc.c

@@ -1613,7 +1613,7 @@ static int mxs_lradc_probe(struct platform_device *pdev)
 			 * of the array.
 			 */
 			scale_uv = ((u64)lradc->vref_mv[i] * 100000000) >>
-				   (iio->channels[i].scan_type.realbits - s);
+				   (LRADC_RESOLUTION - s);
 			lradc->scale_avail[i][s].nano =
 					do_div(scale_uv, 100000000) * 10;
 			lradc->scale_avail[i][s].integer = scale_uv;