mac80211: restrict to AP in outgoing interface heuristic

We try to find the correct outgoing interface for injected frames
based on the TA, but since this is a hack for hostapd 11w, restrict
the heuristic to AP mode interfaces. At some point we'll add the
ability to give an interface index in radiotap or so and just
remove this heuristic again.

Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Cc: stable@kernel.org [2.6.28.x]
Signed-off-by: John W. Linville <linville@tuxdriver.com>