power: bq25890: fix ADC mode configuration

Datasheet describes two modes for reading ADC measurements:
1. continuous, 1 Hz - enabled and started by CONV_RATE bit
2. one-shot - triggered by CONV_START bit

In continuous mode, CONV_START is read-only and signifies an ongoing
conversion.

Change the code to follow the datasheet and really disable continuous
mode for power saving.

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