Register/unregister the expander devices to SCSI Transport Layer(STL)
whenever the corresponding expander is added/removed from topology.

Link: https://lore.kernel.org/r/20220804131226.16653-10-sreekanth.reddy@broadcom.comReviewed-by: Himanshu Madhani <himanshu.madhani@oracle.com>
Signed-off-by: Sreekanth Reddy <sreekanth.reddy@broadcom.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>