mac80211: fix WARN_ON in the new work code

ieee80211_work_rx_mgmt currently enqueues various management frames,
including deauth and disassoc frames, however the function
ieee80211_work_rx_queued_mgmt does not handle these, as they should
only occur if the AP is buggy. It does emit a WARN_ON when this happens
though, and several users have reported such instances.
Fix the WARN_ON by not queueing such frames in the first place.

Signed-off-by: Felix Fietkau <nbd@openwrt.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>