iio: adc: ad7606_spi: fix reg write value mask

commit 89944d8 upstream.

Fix incorrect value mask for register write. Register values are 8-bit,
not 9. If this function was called with a value > 0xFF and an even addr,
it would cause writing to the next register.

Fixes: f2a22e1 ("iio: adc: ad7606: Add support for software mode for ad7616")
Signed-off-by: David Lechner <dlechner@baylibre.com>
Reviewed-by: Angelo Dureghello <adureghello@baylibre.com>
Link: https://patch.msgid.link/20250428-iio-adc-ad7606_spi-fix-write-value-mask-v1-1-a2d5e85a809f@baylibre.com
Cc: <Stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Author: dlech

drivers/iio/adc/ad7606_spi.c

@@ -151,7 +151,7 @@ static int ad7606_spi_reg_write(struct ad7606_state *st,
 	struct spi_device *spi = to_spi_device(st->dev);
 
 	st->d16[0] = cpu_to_be16((st->bops->rd_wr_cmd(addr, 1) << 8) |
-				  (val & 0x1FF));
+				  (val & 0xFF));
 
 	return spi_write(spi, &st->d16[0], sizeof(st->d16[0]));
 }