mac80211: check fast-xmit on station change

Drivers with fast-xmit (e.g. ath10k) running in
AP_VLAN setups would fail to communicate with
connected 4addr stations.

The reason was when new station associates it
first goes into master AP interface. It is not
until later that a dedicated AP_VLAN is created
for it and the station itself is moved there.
After that Tx directed at the station should use
4addr header. However fast-xmit wasn't
recalculated and 3addr header remained to be used.
This in turn caused the connected 4addr stations
to drop packets coming from the AP until some
other event would cause fast-xmit to recalculate
for that station (which could never come).

Signed-off-by: Michal Kazior <michal.kazior@tieto.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>