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 <stefanr@s5r6.in-berlin.de>

drivers/firewire/fw-card.c

@@ -539,7 +539,7 @@ fw_core_remove_card(struct fw_card *card)
 	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);