scsi: cxgb4i: libcxgbi: cxgb4: add T6 iSCSI completion feature

T6 adapters reduce number of completions to host by generating single
completion for all the directly placed(DDP) iSCSI pdus in a sequence.

This patch adds new structure for completion hw cmd (struct
cpl_rx_iscsi_cmp) and implements T6 completion feature.

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