iio: adc: axp20x_adc: remove !! in favor of ternary condition

!!'s behaviour isn't that obvious and sparse complained about it, so
let's replace it with a ternary condition.

Signed-off-by: Quentin Schulz <quentin.schulz@bootlin.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>