RDMA/cxgb4: Remove unneeded assignment

We don't need to assign rpl here, we do that later on.

Signed-off-by: Dan Carpenter <error27@gmail.com>

[ Indeed this assignment makes no sense, since skb is set to NULL a
  couple of lines before.  - Roland ]

Signed-off-by: Roland Dreier <rolandd@cisco.com>