power: supply: axp288_fuel_gauge: Optimize get_current()

First check the discharge current, and when that is non 0 use that without
also checking the charge current (which will be 0 then). This makes
get_current() do only 1 i2c read instead of 2 when on battery.

This is esp. important given the pmic i2c bus mutex stuff used on boards
with an axp288 because the SoC's own punit also may access the axp288,
which makes i2c accesses more expensive then normal.

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