iio: adc: axp20x_adc: Add missing sentinel to AXP717 ADC channel maps

In the Linux kernel, the following vulnerability has been resolved: iio: adc: axp20x_adc: Add missing sentinel to AXP717 ADC channel maps The AXP717 ADC channel maps is missing a sentinel entry at the end. This causes a KASAN warning. Add the missing sentinel entry.