cfg80211: support configuring vif mac addr on create

This is useful when creating virtual interfaces.
Keeps udev from mucking with things it shouldn't, since
the default MAC is never seen by udev when specified on
the cmd-line during creation.

Signed-off-by: Ben Greear <greearb@candelatech.com>
[check for feature flag in nl80211 to force drivers to set it]
Signed-off-by: Johannes Berg <johannes.berg@intel.com>