wifi: mac80211: accept protected frames for NAN device

Some frames sent to the NAN device may be protected, such as
protected action frames (in particular protected dual of
public action).

Accept robust management frames except disassoc on the NAN
device, and clean up the code a little bit.

Change-Id: Ia25b2e82b250058fefa179c39327d9c8b3c3cd62
Signed-off-by: Avraham Stern <avraham.stern@intel.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Miri Korenblit <miriam.rachel.korenblit@intel.com>