iio: at91-sama5d2: Fix incorrect sign extension

Use scan_type when processing raw data which also fixes that the sign
extension was from the wrong bit.

Use channel definition as root of trust and replace constant
when reading elements directly using the raw sysfs attributes.

Fixes: 6794e23f ("iio: adc: at91-sama5d2_adc: add support for oversampling resolution")
Signed-off-by: Gwendal Grignou <gwendal@chromium.org>
Reviewed-by: Eugen Hristev <eugen.hristev@microchip.com>
Cc: <Stable@vger.kernel.org>
Link: https://lore.kernel.org/r/20211104082413.3681212-9-gwendal@chromium.org


Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>