scsi: cxgb4i: validate tcp sequence number only if chip version <= T5

T6 adapters generates DDP completion message on receiving all iSCSI pdus in
a sequence. Because of this, driver can not keep track of tcp sequence
number for T6 adapters.

Signed-off-by: Varun Prakash <varun@chelsio.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>