firewire: cdev: reference-count client instances

The lifetime of struct client instances must be longer than the lifetime
of any client resource.

This fixes a possible race between fw_device_op_release and transaction
completions.  It also prepares for new ioctls for isochronous resource
management which will involve delayed processing of client resources.

Signed-off-by: Stefan Richter <stefanr@s5r6.in-berlin.de>
Reviewed-by: David Moore <dcm@acm.org>