ofctl_v1_0: fix stats timeout handling

This patch fixes a issue reported by https://github.com/osrg/ryu/issues/7

Signed-off-by: OHMURA Kei <ohmura.kei@lab.ntt.co.jp>
Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>