RDMA/mlx5: Don't overwrite action if already set

We support only a single action type per flow rule, in case the user passes
the same type of flow actions fail the flow creation.

Signed-off-by: Mark Bloch <markb@mellanox.com>
Reviewed-by: Yishai Hadas <yishaih@mellanox.com>
Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>