firewire: sbp2: fold two functions into one

sbp2_release_target() is folded into its primary user, sbp2_remove().
The only other caller, a failure path in sbp2_probe(), now uses
sbp2_remove().  This adds unnecessary cancel_delayed_work_sync() calls
to that failure path but results in less code and text.

Signed-off-by: Stefan Richter <stefanr@s5r6.in-berlin.de>