power: supply: tps65217: Use 'poll_task' on unloading the module

Use the task_struct variable for running polling thread. If polling task
is activated, then use it to stop running thread.
This is a preceding step of supporting two interrupts of TPS65217 charger,
so checking single IRQ number is not appropriate when the module is removed.

Signed-off-by: Milo Kim <woogyom.kim@gmail.com>
Signed-off-by: Sebastian Reichel <sre@kernel.org>