RDMA/cxgb4: Use ULP_MODE_TCPDDP

Set the ULP mode for initial RDMA connection setup to the proper DDP
mode.  This avoids wasting some HW resources while in streaming mode.

Signed-off-by: Steve Wise <swise@opengridcomputing.com>
Signed-off-by: Roland Dreier <roland@purestorage.com>