mac80211: check fast-xmit if IBSS STA QoS changed

When an IBSS station gets QoS enabled after having been added,
check fast-xmit to make sure the QoS header gets added to the
cache properly and frames can go out with QoS and higher rates.

Reported-by: Janusz Dziedzic <janusz.dziedzic@tieto.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>