RDMA/mlx5: Enable reformat on NIC RX if supported

A L3_TUNNEL_TO_L2 decap flow action requires to enable the encap bit on
the flow table, enable it if supported. This will allow to attach those
flow actions to NIC RX steering. We don't enable if running on a
representor.

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