iwlwifi: mvm: flush queue before deleting ROC

Before deleting a time event (remain-on-channel instance), flush
the queue so that frames cannot get stuck on it. We already flush
the AUX STA queues, but a separate station is used for the P2P
Device queue.

Cc: stable@vger.kernel.org
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>