wifi: mac80211: fix ieee80211_drop_unencrypted_mgmt return type/value

Somehow, I managed to botch this and pretty much completely break
wifi. My original patch did contain these changes, but I seem to
have lost them before sending to the list. Fix it now.

Reported-and-tested-by: Kalle Valo <kvalo@kernel.org>
Fixes: 6c02fab7 ("wifi: mac80211: split ieee80211_drop_unencrypted_mgmt() return value")
Signed-off-by: Johannes Berg <johannes.berg@intel.com>