[SCSI] libsas: fix bug for vacant phy

This patch fix bug reported by Chuck. And this new version incorporate comments
from Hannes. Please consider to include it into mainline.

Signed-off-by: Jack Wang <jack_wang@usish.com>
Signed-off-by: Lindar Liu <lindar_liu@usish.com>
Tested-by: Chuck Tuffli <Chuck_Tuffli@pmc-sierra.com>
Signed-off-by: James Bottomley <James.Bottomley@suse.de>