RDMA/mlx5: Don't rely on FW to set zeros in ECE response

The FW returns zeros in case feature is not enabled, but it is better to
have the capability check and ensure that returned result is cleared.

Fixes: 3e09a427 ("RDMA/mlx5: Get ECE options from FW during create QP")
Link: https://lore.kernel.org/r/20200602125548.172654-3-leon@kernel.org


Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>