drivers:iio:tla2518: Fix wrong selected mode: value for ALL_GPIOS was wrong

For some reason the define TLA2518_PIN_ALL_ADC was set to 0xFF what means gpio mode is selected. This somehow was not noted, as the ADC generally still did work. But it drains too much current, leading to wrong measurements. This sets the config to the correct value of 0.

PBB-47

assigned to @jonas.hoeppner

merged

Integration merge request: https://git.seco.com/seco-ne/yocto/custom/pbb/manifest/-/merge_requests/172