power: supply: axp288_fuel_gauge: Read 15 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 15 bit value in one consecutive read.

This fixes charge_full reporting 3498768 on some reads and 3354624 one
other reads on my tablet (for the 3354624 value the raw LSB is 0x00).

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