power: supply: axp288_charger: Pick lower input current limit not higher

The code before this commit would pick 900 mA when asking for an input
current limit of 600mA, rather then 500 mA, not good.

While touching almost all code using the silly xxxMA defines anyways,
also get rid of these simply typing out the numbers and switch the
unit to uA as that is the psy class standard unit for currents.

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>