mac80211: drop spoofed packets in ad-hoc mode (fixes #12427)

Signed-off-by: Felix Fietkau <nbd@openwrt.org>

Backport of r38013

git-svn-id: svn://svn.openwrt.org/openwrt/branches/attitude_adjustment@38014 3c298f89-4303-0410-b956-a3cf2f4a3e73