power: supply: ab8500_charger: Restrict ADC retrieveal

The AB8505 only has two ADC channels: the voltage and current
provided from VBUS (USB). It does not support AC charging at all.
Make sure we don't try to retrieve the non-existing channels.

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>