cxgb4: Support firmware rdma write completion work request.

If FW supports RDMA WRITE_COMPLETION functionality, then advertise that
to the ULDs. This will be used by iw_cxgb4 to allow WRITE_COMPLETION
work requests.

Signed-off-by: Potnuri Bharat Teja <bharat@chelsio.com>
Signed-off-by: Raju Rangoju <rajur@chelsio.com>
Signed-off-by: Ganesh Goudar <ganeshgr@chelsio.com>
Signed-off-by: Steve Wise <swise@opengridcomputing.com>
Signed-off-by: David S. Miller <davem@davemloft.net>