scsi: target: cxgbit: Check connection state before issuing hardware command

Current code does not check connection state before issuing
header/data digest offload and DDP page size setup hardware command.

Add a connection state check to issue hardware command only
if connection is in established state.

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