[SCSI] mpt2sas: Fix to use sas device list instead of enclosure list for _transpor_get_enclosure_identifier.

Enclosure_identifier not being returned by mpt2sas
The driver exports callback function to the sas transport layer
for obtaining the enclosure logical id.  This function is called
_transport_get_enclosure_identifier.  The driver was searching
the wrong list for the enclosure_identifier.  The driver should be
searching the sas device list instead of enclosure list.  The
sas address that is passed to the driver is for the end device, not
enclosure.
Signed-off-by: Kashyap Desai <kashyap.desai@lsi.com>
Signed-off-by: James Bottomley <James.Bottomley@suse.de>