power: supply: 88pm860x: simplify using devm

Use devm variants for requesting threaded IRQ and for power-supply
registration. Clean up error path and remove the .remove-callback.

Signed-off-by: Matti Vaittinen <matti.vaittinen@fi.rohmeurope.com>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>