net/mlx5: Move eswitch port metadata devlink param to flow eswitch code

Move the param registration and handling code into the eswitch offloads
code as they are related to each other. No point in having the
devlink param registration done in separate file.

Signed-off-by: Jiri Pirko <jiri@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>