lp8788-charger: Fix ADC channel names

The name of ADC channel is configurable in the platform side. This name is
referenced in the IIO consumer driver. To get the IIO channel, specific
name in the platform data is used as an parameter of the
iio_channel_get(). Thus, lp8788_adc_id platform data are replaced with
specific names.

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