power: supply: max8997_charger: Use devm_work_autocancel()

Use devm_work_autocancel() instead of hand writing it.
It saves a few lines of code.

Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>