power: supply: max17040: simplify POWER_SUPPLY_PROP_ONLINE

The driver was reporting POWER_SUPPLY_PROP_ONLINE via platform data
functions or '1' if no platform data was provided.  Since there are no
platforms using the driver with platform data (no board files with the
driver), the online property can be simplified to always return '1'.

Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@canonical.com>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>