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>