ANDROID: net: wireless: Add module_param(mac_prefix) to mac80211_hwsim

By default mac80211_hwsim creates N radios with
MAC addresses like '02:00:00:nn:nn:00' where nnnn
is incremented.

We (Android Studio Emulator) need a way to have unique
MAC addresses, mac_prefix provides a way to specify
two octets in MAC address, i.e. '02:xx:xx:nn:nn:00'
to make it unique between emulator instances.

This is temporary solution and should be replaced
by HWSIM_CMD_NEW_RADIO with HWSIM_ATTR_PERM_ADDR,
the work is tracked in b/149778407.

Bug: 150388981
Bug: 149778407
Bug: 147493341
Test: check if wifi works on emulator
Signed-off-by: Roman Kiryanov <rkir@google.com>
Change-Id: Ibab0de6cb720d55c7be0b96e40ec70e221e04d7f