hostapd: Only update already-set beacons when processing OLBC

hostapd: Only update already-set beacons when processing OLBC

When hostapd is about to start an AP using HT40, it starts a scan to
check whether it is allowed or not. If OLBC is detected before the
scan has completed, it sets the beacons prematurely.

To fix this, instead of setting all beacons when OLBC is detected,
only update the ones that have already been started.

Signed-off-by: Luciano Coelho <coelho@ti.com>