In IBSS mode, stop calling the driver's newassoc() each time a probe

In IBSS mode, stop calling the driver's newassoc() each time a probe
response is received from a node. Only call it once when the node is
initially discovered, as used to be the case before r1.4 of this file.
Asking the driver to set up its per-node private state once is enough.
Also remove an outdated comment.
ok sthen deraadt jsg