iio: adc: ad7606_spi: fix reg write value mask

iio: adc: ad7606_spi: fix reg write value mask

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: f2a22e1e172f ("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>