iwlwifi: mvm: remove is_data_qos variable in TX

"is_data_qos == true" is equivalent to "tid < IWL_MAX_TID_COUNT"
since tid is only assigned (and range-checked) in that case.

This removes a (harmless) smatch warning that occurs because it
can't seem to follow the above logic from the code.

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