wifi: nl80211: Add POWERED_ADDR_CHANGE feature

Add a new extended feature bit signifying that the wireless hardware
supports changing the MAC address while the underlying net_device is
powered. Note that this has a different meaning from
IFF_LIVE_ADDR_CHANGE as additional restrictions might be imposed by
the hardware, such as:

 - No connection is active on this interface, carrier is off
 - No scan is in progress
 - No offchannel operations are in progress

Signed-off-by: James Prestwood <prestwoj@gmail.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>