mac80211: don't warn when stopping VLAN with stations

Stations assigned to an AP_VLAN type interface are flushed
when the interface is stopped, but then we warn about it.
Suppress the warning since there's nothing else that would
ensure those stations are already removed at this point.

Reported-by: Jouni Malinen <j@w1.fi>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>