mac80211: use different MAC addresses for virtual interfaces

Drivers can now advertise to cfg80211 that they have
multiple MAC addresses reserved for a device, but we
don't currently make use of that in mac80211.

Change that and assign different addresses to new
virtual interfaces (if addresses are available) in
order to make it easier for users to use multiple
virtual interfaces; they no longer need to always
assign a new MAC address manually.

Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>