hostapd: Process management frames only once per BSS

hostapd: Process management frames only once per BSS

At least in nl80211, broadcast management frames like Probe Request
frames, may be processed multiple times per BSS if multi-BSS is active
and NL80211_CMD_FRAME event is used to deliver them. In the case of
Probe Request frames, hostapd will create multiple redundant Probe
Response frames which are problematic when many BSS are on one channel.

This problem is caused by driver_nl80211 generating an event for
wpa_supplicant_event() for each BSS, and hostapd_mgmt_rx() calls
ieee802_11_mgmt() for each BSS, too.

Fix this by processing broadcast events only for the BSS the driver
intended to. The behavior is not changed for drivers not setting a BSS.

Signed-hostap: Simon Wunderlich <simon@open-mesh.com>