iwlwifi: don't configure a txq that is being disabled

This is not needed, we just need to tell the SCD not to use
that queue. We will reconfigure that queue when we will use
it again.

Clean up a bit the code on the way.

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