cfg80211: consider VHT opmode on station update

Currently, this attribute is only fetched on station addition, but
not on station change. Since this info is only present in the assoc
request, with full station state support in the driver it cannot be
present when the station is added.

Thus, add support for changing the VHT opmode on station update if
done before (or while) the station is marked as associated. After
this, ignore it, since it used to be ignored.

Signed-off-by: Beni Lev <beni.lev@intel.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>