driver: nl80211: add .get_mac_addr() callback for P2P-Device

For P2P-Device the mac address is determined upon .init2(). The
wpa_s for this interface retrieves this address in
wpa_supplicant_update_mac_addr() using the .get_mac_addr() callback.

Signed-off-by: Arend van Spriel <arend@broadcom.com>