mlxsw: spectrum_trap: Move struct definition out of header file

'struct mlxsw_sp_trap_policer_item' is only used in one file, so move it
there.

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