Fix the 'scan hang' issue.

When requesting a scan and one is already in progess, e.g. while in scan
state, we happily wait for a scan done notification. Though, this
notification might never be sent, e.g. if we are trying to find a network
to associate to and there is none. Instead of always waiting for a
notification just do so if a new scan has been started. For both cases the
scan cache is used to report available networks even if the content might
not be fresh.

MFC after: 1 month