mlxsw: spectrum_router: Compare only trees which are in use during tree get

Only trees which are in use should be compared to requested prefix usage.

Fixes: 53342023 ("mlxsw: spectrum_router: Implement LPM trees management")
Signed-off-by: Jiri Pirko <jiri@mellanox.com>
Reviewed-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>