[SCSI] mpt2sas: Do not set sas_device->starget to NULL from the slave_destroy callback when all the LUNS have been deleted

If the sas_device->starget to NULL from slave_destroy callback for LUN=1
even though LUN=0 exist, results in entire target getting deleted.
To resolve the issue, the driver should only set sas_device->starget to
NULL when all the LUNS have been deleted from the slave_destroy.
Signed-off-by: NNagalakshmi Nandigama <nagalakshmi.nandigama@lsi.com>
Signed-off-by: NJames Bottomley <JBottomley@Parallels.com>