firewire: warn on unfinished transactions during card removal

After card->done and card->work are completed, any remaining pending
request would be a bug. We cannot safely complete a transaction at
that point anymore.

IOW card users must not drop their last fw_card reference (usually
indirect references through fw_device references) before their last
outbound transaction through that card was finished.

Signed-off-by: Stefan Richter <[e-mail]>

drivers/firewire/fw-card.c

···
 	wait_for_completion(&card->done);
 
 	cancel_delayed_work_sync(&card->work);
-	fw_flush_transactions(card);
+	WARN_ON(!list_empty(&card->transaction_list));
 	del_timer_sync(&card->flush_timer);
 }
 EXPORT_SYMBOL(fw_core_remove_card);