mlxsw: spectrum: Use same switch case for identical groups

Trap groups that use the same policer settings can share the same switch
case.

Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Reviewed-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>