[SCSI] scsi_transport_sas; fix user_scan

the user_scan() callback currently has the potential to identify the
wrong device in the presence of expanders.  This is because it finds
the first device with a matching target_id, which might be an
expander.  Fix this by making it look specifically for end devices.

Signed-off-by: James Bottomley <James.Bottomley@SteelEye.com>