Drivers may need to track which vif is using VHT MU-MIMO.
Move the flag indicationg the ownership of MU_MIMO to
ieee80211_vif.

Signed-off-by: Sara Sharon <sara.sharon@intel.com>
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>