iwlwifi: mvm: wait for the flushed queue only

The function flushed only agg queue and no more traffic
is queued on it.
However, it waits for all queues to empty, which is not
necessary and may take more time.

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