iw_cxgb4: reflect the original WR opcode in drain cqes

The flush/drain logic was not retaining the original wr opcode in
its completion.  This can cause problems if the application uses
the completion opcode to make decisions.

Use bit 10 of the CQE header word to indicate the CQE is a special
drain completion, and save the original WR opcode in the cqe header
opcode field.

Fixes: 4fe7c296 ("iw_cxgb4: refactor sq/rq drain logic")

Cc: stable@vger.kernel.org
Signed-off-by: Steve Wise <swise@opengridcomputing.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>