mac80211_hwsim: Support a hw scan request for a specific BSSID

If the hw scan request 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>