power: bq25890: use proper CURRENT_NOW property for I_BAT

Charge Current is more apropriately reflected by CURRENT_NOW property
(measured current) than CONSTANT_CURRENT_VOLTAGE (configured CC-phase
current limit). Fix the reference and make the sign reflect direction
of the current.

Signed-off-by: Michał Mirosław <mirq-linux@rere.qmqm.pl>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>