power: supply: axp20x_battery: properly report current when discharging

[ Upstream commit d4f408cdcd26921c1268cb8dcbe8ffb6faf837f3 ]

As stated in [1], negative current values are used for discharging
batteries.

AXP PMICs internally have two different ADC channels for shunt current
measurement: one used during charging and one during discharging.
The values reported by these ADCs are unsigned.
While the driver properly selects ADC channel to get the data from,
it doesn't apply negative sign when reporting discharging current.

[1] Documentation/ABI/testing/sysfs-class-power

Signed-off-by: Evgeny Boger <boger@wirenboard.com>
Acked-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>