Extend random MAC address support to allow OUI to be kept

Extend random MAC address support to allow OUI to be kept

mac_addr=2 and preassoc_mac_addr=2 parameters can now be used to
configure random MAC address to be generated by maintaining the OUI part
of the permanent MAC address (but with locally administered bit set to
1). Other than that, these values result in similar behavior with
mac_addr=1 and preassoc_mac_addr=1, respectively.

Signed-off-by: Jouni Malinen <j@w1.fi>