scsi_transport_sas: add function to get SAS endpoint address

For a device known to be SAS connected, this will return the endpoint
address.  This is useful for getting the SAS address of SATA devices.

Reviewed-by: Hannes Reinecke <hare@suse.com>
Signed-off-by: James Bottomley <James.Bottomley@HansenPartnership.com>