scsi: dpt_i2o: stop using scsi_unregister

dpt_i2o doesn't use scsi_register, so it should not use scsi_unregister
either.  Also refactor the module exit path to make a little more sense.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>