mlxsw: spectrum_trap: Move layer 3 exceptions to exceptions trap group

The layer 3 exceptions are still subject to the same trap policer, so
nothing changes, but user space can choose to assign a different one.

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