mac80211: Add validity check for beacon_crc value

On association to an AP, after receiving beacons, the beacon_crc value is set.
The beacon_crc value is not reset in disassociation, but the BSS data may be
expired at a later point. When associating again, it's possible that a
beacon for the AP is not received, resulting in the beacon_ies to remain NULL.

After association, further beacons will not update the beacon data, as the
crc value of the beacon has not changed, and the beacon_crc still holds a
value matching the beacon. The beacon_ies will remain forever null.

One of the results of this is that WLAN power save cannot be entered, the STA
will remain foreven in active mode.

Fix this by adding a validation flag for the beacon_crc, which is cleared on
association.

Signed-off-by: Juuso Oikarinen <juuso.oikarinen@nokia.com>
Acked-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>