net/mlx5: Move flow steering devlink param to flow steering code

Move the param registration and handling code into the flow steering
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>
Reviewed-by: Jacob Keller <jacob.e.keller@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>