ACPI: bus: Ensure that notify handlers are not running after removal

Currently, acpi_device_remove_notify_handler() may return while the
notify handler being removed is still running which may allow the
module holding that handler to be torn down prematurely.

Address this issue by making acpi_device_remove_notify_handler() wait
for the handling of all the ACPI events in progress to complete before
returning.

Fixes: 5894b0c4 ("ACPI / scan: Move bus operations and notification routines to bus.c")
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>