[SCSI] scsi_transport_sas: add support for transport layer retries (TLR)

The mpt2sas driver wants to use transport layer retries (TLR) so the
simplest thing to do seems to be to add the enabling flags and checks
to the SAS transport class, since they're a SAS specific protocol
feature.

Signed-off-by: James Bottomley <James.Bottomley@suse.de>