iwlwifi: mvm: Flush non STA TX queues

When starting wowlan mac80211 requests flush w/o vif
and we ignore this request. As a result some packets
stay stuck in the queue and it may end up with a queue
hang.

Allow the driver to flush queues even if station isn't
specified.

Signed-off-by: David Spinadel <david.spinadel@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>