net/mlx5: DR, Prepare sending new WQE type

The send engine should be ready to handle more opcodes
in addition to RDMA_WRITE/RDMA_READ.

Signed-off-by: Yevgeny Kliteynik <kliteyn@nvidia.com>
Reviewed-by: Alex Vesker <valex@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>