scsi: ibmvscsis: Properly deregister target sessions

The driver currently doesn't properly deregisters target sessions
completely, so this will address that.

Signed-off-by: Bryant G. Ly <bryantly@linux.vnet.ibm.com>
Signed-off-by: Michael Cyr <mikecyr@linux.vnet.ibm.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>