power: supply: axp288_fuel_gauge: Read 12 bit values 2 registers at a time

In order for the MSB -> LSB latching to work correctly we must read the
2 8 bit registers of a 12 bit value in one consecutive read.

This fixes voltage_ocv reporting inconsistent values on my tablet.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Sebastian Reichel <sre@kernel.org>