cxgb4: fix abort_req_rss6 struct

Remove the incorrect WR_HDR field which can cause a misinterpretation
of ABORT CPL by ULDs, such as iw_cxgb4.

Fixes: a3cdaa69 ("cxgb4: Adds CPL support for Shared Receive Queues")
Signed-off-by: Steve Wise <swise@opengridcomputing.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>