ofproto: Do not add flow on flow mod if new_cookie is UINT64_MAX

ofproto: Do not add flow on flow mod if new_cookie is UINT64_MAX

Internally a new_cookie value UINT64_MAX is used for
an OpenFlow 1.2 flow mod whose command is not Add.
Open Flow 1.2 does not permit adding flows for such commands.
Also, UINT64_MAX is a prohibited cookie value, and the
existing code created a flow with that value as for the cookie.

Signed-off-by: Simon Horman <horms@verge.net.au>
Signed-off-by: Ben Pfaff <blp@nicira.com>