iwlagn: use IWL_MAX_TID_COUNT instead of TID_MAX_LOAD_COUNT

We track the load only on 8 TIDs, previously this
was TID_MAX_LOAD_COUNT. Since IWL_MAX_TID_COUNT
is now 8 as well, use that to make the code more
easily understandable.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Wey-Yi Guy <wey-yi.w.guy@intel.com>