[SCSI] cleanup usages of scsi_complete_async_scans

Now that scsi registers its async scan work with the async subsystem,
wait_for_device_probe() is sufficient for ensuring all scanning is
complete.

Signed-off-by: Dan Williams <dan.j.williams@intel.com>
Cc: Rafael J. Wysocki <rjw@sisk.pl>
Tested-by: Eldad Zack <eldad@fogrefinery.com>
Signed-off-by: James Bottomley <JBottomley@Parallels.com>