[SCSI] libsas: fix sas_find_bcast_phy() in the presence of 'vacant' phys

If an expander reports 'PHY VACANT' for a phy index prior to the one
that generated a BCN libsas fails rediscovery.  Since a vacant phy is
defined as a valid phy index that will never have an attached device
just continue the search.

Cc: <stable@vger.kernel.org>
Signed-off-by: Thomas Jackson <thomas.p.jackson@intel.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>
Signed-off-by: James Bottomley <JBottomley@Parallels.com>