mac80211: Support a scan request for a specific BSSID

If the cfg80211 scan trigger operation specifies a single BSSID, use
that value instead of the wildcard BSSID in the Probe Request frames.

Signed-off-by: Jouni Malinen <jouni@qca.qualcomm.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>