power: supply: bq25890: Factor out chip state update

Pull the chip state and ADC conversion update functionality out into
separate function, so it can be reused elsewhere in the driver. This
is a preparatory patch, no functional change.

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