IB/mlx4: Fix opcode returned in RDMA read completion

Current code has a cut-and-paste error and returns IB_WC_SEND when it
should return IB_WC_RDMA_READ.

Signed-off-by: Vu Pham <vu@mellanox.com>
Signed-off-by: Roland Dreier <rolandd@cisco.com>