cfg80211: allow setting TXQ parameters only in AP mode

In other modes the parameters should not be set.
Right now, mac80211 will set them, even if the
user asked for setting them on VLANs which the
driver doesn't know about, causing all kinds of
trouble.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>