mac80211: fix TX aggregation TID struct leak

Ben reports that kmemleak is saying TX aggregation TID
structs are leaked. Given his workload, I suspect that
they're leaked because stations are destroyed before
their aggregation sessions get a chance to start. Fix
this by simply freeing structs that are not used yet.

Reported-by: Ben Greear <greearb@candelatech.com>
Tested-by:  Ben Greear <greearb@candelatech.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>