mac80211: fix kernel crash on netdev unregister on newer 3.14 kernels

Based on patch by Mark Milburn

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

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@43224 3c298f89-4303-0410-b956-a3cf2f4a3e73