mac80211: Self-protected management frames are not robust

They may contain encrypted information elements (as AMPE frames do)
but they are not encrypted.

Signed-off-by: Thomas Pedersen <thomas@cozybit.com>
Signed-off-by: Javier Cardona <javier@cozybit.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>