power: supply: bq25890: Clean up POWER_SUPPLY_PROP_CONSTANT_CHARGE_CURRENT

Clean up misuse of POWER_SUPPLY_PROP_CONSTANT_CHARGE_CURRENT and
POWER_SUPPLY_PROP_CONSTANT_CHARGE_CURRENT_MAX and document what
exactly each value means.

The POWER_SUPPLY_PROP_CONSTANT_CHARGE_CURRENT content is newly read
back from hardware, while POWER_SUPPLY_PROP_CONSTANT_CHARGE_CURRENT_MAX
is reported as the maximum value set in DT.

Reviewed-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Marek Vasut <marex@denx.de>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>