sbp2: did not clean up after scsi_add_device() failed

If scsi_add_device() at the end of sbp2_start_device() fails, e.g. due to
transport errors during SCSI inquiry, sbp2 needs to log out of the device
and release all associated resources.

Signed-off-by: Stefan Richter <stefanr@s5r6.in-berlin.de>
Signed-off-by: Jody McIntyre <scjody@modernduck.com>