power: bq25890: Enable continuous conversion for ADC at charging

Instead of one shot run of ADC at beginning of charging, run continuous
conversion to ensure that all charging-related values are monitored
properly (input voltage, input current, themperature etc.).

Signed-off-by: Yauhen Kharuzhy <jekhor@gmail.com>
Reviewed-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>