iwlwifi: add wait for tx queue empty

Now that we have 512 queues, add a wait for single TX
queue to gen2.
This replaces gen1 wait_tx_queues_empty, which was limited
to 32 queues.

Signed-off-by: Sara Sharon <sara.sharon@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>