power: bq24257: Use managed power supply register

Use the devm_* managed version of the function to register the power
supply and remove the associated unregister function. This will simplify
error handling moving forward as it allows the unregister to happen
automatically. It also saves a few lines of code.

As this changes the order of putting the bq24257 into reset vs.
unregistering the power-supply during driver remove re-tested various
driver unload scenario to make sure that this doesn't cause any
unintended side effects such as erroneous interrupts.

Signed-off-by: Andreas Dannenberg <dannenberg@ti.com>
Reviewed-by: Krzysztof Kozlowski <k.kozlowski@samsung.com>
Signed-off-by: Sebastian Reichel <sre@kernel.org>