power: supply: axp288_fuel_gauge: Add dev helper var to probe()

Add a dev local variable to probe() as shortcut for &pdev->dev, this is
a preparation change for making more use of devm managed resources.

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