power: supply: axp288_fuel_gauge: Fix external_power_changed race

In the Linux kernel, the following vulnerability has been resolved:

power: supply: axp288_fuel_gauge: Fix external_power_changed race

fuel_gauge_external_power_changed() dereferences info->bat,
which gets sets in axp288_fuel_gauge_probe() like this:

info->bat = devm_power_supply_register(dev, &fuel_gauge_desc, &psy_cfg);

As soon as devm_power_supply_register() has called device_add()
the external_power_changed callback can get called. So there is a window
where fuel_gauge_external_power_changed() may get called while
info->bat has not been set yet leading to a NULL pointer dereference.

Fixing this is easy. The external_power_changed callback gets passed
the power_supply which will eventually get stored in info->bat,
so fuel_gauge_external_power_changed() can simply directly use
the passed in psy argument which is always valid.