power: supply: max14656: fix potential use-after-free

Explicitly cancel/sync the irq_work delayed work, otherwise
there's a chance that it will run after the device is removed,
which would result in a use-after-free.

Note that cancel/sync should happen:
- after irq's have been disabled, as the isr re-schedules the work
- before the power supply is unregistered, because the work func
    uses the power supply handle.

Cc: Alexander Kurz <akurz@blala.de>
Signed-off-by: Sven Van Asbroeck <TheSven73@gmail.com>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>