iwlwifi: dvm: move ADD_STA response handling to sync command

This driver currently has some very confusing ADD_STA response handling
that runs asynchronously in the background for all of the commands, but
is only really necessary for synchronous ones (the really asynchronous
ones can only be done for already existing stations), and for the sync
ones it actually waits for the RX handler to return a status code.

Rework this to keep the debug printing in the handler, but do the code
that's supposed to have an effect only for sync commands in the command
sending function.

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