firewire: cdev: remove unneeded idr_find() from complete_transaction()

Outbound transactions are never aborted with release_client_resource(),
so it is not necessary for complete_transaction() to check whether the
resource is still registered.  Only shutdown_resource() can abort such
an transaction, and this is already handled with the in_shutdown check.

Signed-off-by: Clemens Ladisch <clemens@ladisch.de>
Signed-off-by: "Stefan Richter" <stefanr@s5r6.in-berlin.de>