mac80211: Do not generate CQM events based on first Beacon frames

The signal strength value in a single RX frame is not that reliable,
so it is better to delay start of CQM events until there is a real
average signal strength from more than a single Beacon frame
available.

Signed-off-by: Jouni Malinen <j@w1.fi>
Signed-off-by: John W. Linville <linville@tuxdriver.com>