mac80211: correct MU-MIMO monitor follow functionality

The MU-MIMO monitor follow functionality is broken because it
doesn't clear the MU-MIMO owner even if both follow features
are disabled. Fix that, and while at it move the code into a
new helper function. Call this also when creating a new monitor
interface to prepare for an upcoming cfg80211 change allowing
that.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>