Our sensor test requires both reading and writing from a sensor's
QOM property. So we need to make the input of ADC module R/W instead
of write only for that to work.

Signed-off-by: Hao Wu <wuhaotsh@google.com>
Reviewed-by: Titus Rwantare <titusr@google.com>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Message-id: 20220714182836.89602-5-wuhaotsh@google.com
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>