power: supply: axp288_fuel_gauge: Get iio-channels once during boot

Get iio-channels once during boot, delaying the probe if the axp288_adc
drivers has not loaded yet, instead of getting them on demand each time
we need them.

This fixes the following errors in dmesg:

axp288_fuel_gauge axp288_fuel_gauge: ADC charge current read failed:-19

Which were caused by the ondemand iio-channel read code not finding the
channel when the axp288_adc driver had not loaded yet.

Reviewed-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.co.uk>