iwlwifi: dvm: drop VO packets when mac80211 tells us to

mac80211 now informs the driver when to drop the packets
upon flush(). This will happen before disconnecting, or
before we shut down the interface. We can now rely on this
to drop all the packets including the VO queues.
When mac80211 sets drop to false, wait for all the queues
to be empty.

Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>