net/mlx5: E-Switch, use new cap as condition for mpls over udp

Use tunnel_stateless_mpls_over_udp instead of
MLX5_FLEX_PROTO_CW_MPLS_UDP since new devices have native support for
mpls over udp and do not rely on flex parser.

Signed-off-by: Eli Cohen <elic@nvidia.com>
Reviewed-by: Roi Dayan <roid@nvidia.com>
Reviewed-by: Mark Bloch <mbloch@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>