scsi: libfc: don't fail sequence abort for completed exchanges

If a sequence should be aborted the exchange might already
be completed (eg if the response is still queued in the rx
queue), so this shouldn't considered as an error.

Signed-off-by: Hannes Reinecke <hare@suse.com>
Acked-by: Johannes Thumshirn <jth@kernel.org>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>