controller/ofp_handler: improve version negotiation

- fix string concatenation. Needs surrounding paren.
- more version checks
- make hello_failed private. (adding "_" prefix)

Signed-off-by: Isaku Yamahata <yamahata@valinux.co.jp>
Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>