lp8788-charger: Use consumer device name on setting IIO channels

To get the ADC value for the battery voltage and temperature, LP8788 ADC
driver is used. LP8788 charger driver is the consumer of LP8788 ADC
driver. Thus, specific ADC driver name is required on getting the channel
using iio_channel_get().

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