wifi: mac80211: flush queues on STA removal

When we remove a station, we first make it unreachable,
then we (must) remove its keys, and then remove the
station itself. Depending on the hardware design, if
we have hardware crypto at all, frames still sitting
on hardware queues may then be transmitted without a
valid key, possibly unencrypted or with a fixed key.

Fix this by flushing the queues when removing stations
so this cannot happen.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>