iwlwifi: change TX code to suppress smatch warning

By using a few temporary variables, smatch can track
what's happening and stops complaining that we access
beyond the tid_data array.

This also makes the generated code a bit smaller, so
it's a win all around.

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