mac80211: reject aggregation sessions with non-HT peers

If a peer or some local agent (rate control, ...) decides to start
an aggregation session but doesn't support HT (which also implies
QoS), reject it.

This is mostly a corner case as such peers normally won't try to
use block-ack sessions and rate control wouldn't start them, but
technically QoS stations could request it according to the spec.

However, since drivers don't really support such non-HT sessions
it's better to reject them.

Also, while at it, move the tracing for TX sessions earlier so it
captures the error cases as well.

Reviewed-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>