lp8788-charger: Fix wrong ADC conversion

To get the battery voltage and temperature, IIO ADC functions are used.
LP8788 ADC driver provides RAW and SCALE channel information. This patch
fixes wrong ADC result.

Signed-off-by: Milo(Woogyom) Kim <milo.kim@ti.com>
Reviewed-by Lars-Peter Clausen <lars@metafoo.de>
Acked-by: Jonathan Cameron <jic23@kernel.org>
Signed-off-by: Anton Vorontsov <anton.vorontsov@linaro.org>