iwlwifi: rename wait_for_tx_queues_empty

Rename current wait_tx_queue_empty to wait_tx_queues_empty since
it waits for multiple queues (up to 32).
Next patch will add a wait for single TX queue which is needed for
gen2 to be scalable for 512.

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