datapath-windows: Return success when duplicate flow is added.

If we are trying to insert a flow while there's already a key with the
same flow, return success instead of failure. It can be argued that we
should probably return a transactional error EEXIST, but we'll handle
this in a subsequent commit. I've added a comment to address this later.

Signed-off-by: Nithin Raju <nithin@vmware.com>
Signed-off-by: Ben Pfaff <blp@nicira.com>
Acked-by: Eitan Eliahu <eliahue@vmware.com>