target/cxgbit: Enable DDP for T6 only if data sequence and pdu are in order

Enable DDP for T6 only if DataSequenceInOrder=YES and
DataPDUInOrder=YES to ensure inorder delivery of iSCSI pdus.

Signed-off-by: Varun Prakash <varun@chelsio.com>
Signed-off-by: Bart Van Assche <bart.vanassche@sandisk.com>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>