WNM: Use recent scan results on BSS transition request

WNM: Use recent scan results on BSS transition request

If the last scans are recent (for now, less than ten seconds old), use
them instead of triggering a new scan when a BSS Transition Management
Request frame is received. As a fallback, allow a new scan to be
triggered if no matches were found.

Signed-off-by: Jouni Malinen <j@w1.fi>