iio: adc: ad7606: use realbits for sign-extending in scan_direct

Currently the 'ad7606' driver supports parts with 18 and 16 bits
resolutions.
But when adding support for AD7607 (which has a 14-bit resolution) we
should check for the 'realbits' field, to be able to sign-extend correctly.

Signed-off-by: Alexandru Ardelean <aardelean@baylibre.com>
Reviewed-by: David Lechner <dlechner@baylibre.com>
Link: https://patch.msgid.link/20241025095939.271811-3-aardelean@baylibre.com


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