bq27x00_battery: Fix OOPS caused by unregistring bq27x00 driver

* power_supply_unregister call bq27x00_battery_get_property which
  call bq27x00_battery_poll
* make sure that bq27x00_battery_poll will not call
  schedule_delayed_work again after unregister (which cause OOPS)

Signed-off-by: Pali Rohár <pali.rohar@gmail.com>
Signed-off-by: Anton Vorontsov <cbouatmailru@gmail.com>