net/mlx5: qos: Flesh out element_attributes in mlx5_ifc.h

This is used for multiple purposes, depending on the scheduling element
created. There are a few helper struct defined a long time ago, but they
are not easy to find in the file and they are about to get new members.
This commit cleans up this area a bit by:
- moving the helper structs closer to where they are relevant.
- defining a helper union to include all of them to help
  discoverability.
- making use of it everywhere element_attributes is used.
- using a consistent 'attr' name.

Signed-off-by: Cosmin Ratiu <cratiu@nvidia.com>
Signed-off-by: Tariq Toukan <tariqt@nvidia.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>