iwlwifi: mvm: drop non VO frames when flushing

This change has already been implemented in iwldvm:

commit a260e7b3f0307878b99d57ed1406cf2d497923b8
Author: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Date:   Sun Oct 5 09:11:14 2014 +0300

    iwlwifi: dvm: drop non VO frames when flushing

Since I added the flush() callback implementation in mvm,
we got reports that the queues are stuck while roaming
or suspending.
This commit above helped much for iwldvm, implement the
same behavior for iwlmvm.

CC: <stable@vger.kernel.org> [3.16+]
Fixes: c5b0e7c0 ("iwlwifi: mvm: implement mac80211's flush callback")
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>