iio: addac: ad74413r: Do not reference negative array offsets

Instead of aiming rx_buf at an invalid array-boundary-crossing location,
just skip the first increment. Seen when building with -Warray-bounds:

drivers/iio/addac/ad74413r.c: In function 'ad74413r_update_scan_mode':
drivers/iio/addac/ad74413r.c:843:22: warning: array subscript -4 is below array bounds of 'u8[16]' { aka 'unsigned char[16]'} [-Warray-bounds]
  843 |         u8 *rx_buf = &st->adc_samples_buf.rx_buf[-1 * AD74413R_FRAME_SIZE];
      |                      ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
drivers/iio/addac/ad74413r.c:84:20: note: while referencing 'rx_buf'
   84 |                 u8 rx_buf[AD74413R_FRAME_SIZE * AD74413R_CHANNEL_MAX];
      |                    ^~~~~~

Cc: Lars-Peter Clausen <lars@metafoo.de>
Cc: Michael Hennerich <Michael.Hennerich@analog.com>
Cc: Jonathan Cameron <jic23@kernel.org>
Cc: linux-iio@vger.kernel.org
Fixes: fea251b ("iio: addac: add AD74413R driver")
Reviewed-by: Cosmin Tanislav <cosmin.tanislav@analog.com>
Signed-off-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/20220112203456.3950884-1-keescook@chromium.org
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

drivers/iio/addac/ad74413r.c

@@ -840,7 +840,7 @@ static int ad74413r_update_scan_mode(struct iio_dev *indio_dev,
 {
 	struct ad74413r_state *st = iio_priv(indio_dev);
 	struct spi_transfer *xfer = st->adc_samples_xfer;
-	u8 *rx_buf = &st->adc_samples_buf.rx_buf[-1 * AD74413R_FRAME_SIZE];
+	u8 *rx_buf = st->adc_samples_buf.rx_buf;
 	u8 *tx_buf = st->adc_samples_tx_buf;
 	unsigned int channel;
 	int ret = -EINVAL;
@@ -894,9 +894,10 @@ static int ad74413r_update_scan_mode(struct iio_dev *indio_dev,
 
 		spi_message_add_tail(xfer, &st->adc_samples_msg);
 
-		xfer++;
 		tx_buf += AD74413R_FRAME_SIZE;
-		rx_buf += AD74413R_FRAME_SIZE;
+		if (xfer != st->adc_samples_xfer)
+			rx_buf += AD74413R_FRAME_SIZE;
+		xfer++;
 	}
 
 	xfer->rx_buf = rx_buf;