scsi: cxgb4i: Set completion bit in work request

If SKCBF_TX_FLAG_COMPL flag is set and unacknowledged credits are >= max
credits / 2, set completion bit in work request to reclaim credits.

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