RDMA/mlx5: Return ECE DC support

RDMA/mlx5: Return ECE DC support

The DC QPs are many-to-one QP types that means that first connection will
establish ECE options that coming connections should follow.  Due to this
property, the ECE code was removed between first [1] and second [2] ECE
submissions.

This patch returns the dropped code, because ECE is a property of a
connection and like any other connection users are needed to manage this
data. Allow them to set ECE parameter for DC too and avoid need of having
compatibility flag for the DC ECE.

[1]
https://lore.kernel.org/linux-rdma/20200523132243.817936-1-leon@kernel.org/
[2]
https://lore.kernel.org/linux-rdma/20200525174401.71152-1-leon@kernel.org/

Link: https://lore.kernel.org/r/20200602125548.172654-4-leon@kernel.org
Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>