mac80211: handle station TX latency allocation errors

When the station's TX latency data structures need to be
allocated, handle failures properly and also free all the
structures if there are any other problems.

Move the allocation code up so that allocation failures
don't trigger rate control algorithm calls.

Reported-by: ZHAO Gang <gamerh2o@gmail.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>