mac80211: Disable hw crypto for GTKs on AP VLAN interfaces

mac80211: Disable hw crypto for GTKs on AP VLAN interfaces

When using AP VLAN interfaces, each VLAN interface should be in its own
broadcast domain. Hostapd achieves this by assigning different GTKs to
different AP VLAN interfaces.

However, mac80211 drivers are not aware of AP VLAN interfaces and as
such mac80211 sends the GTK to the driver in the context of the base AP
mode interface. This causes problems when multiple AP VLAN interfaces
are used since the driver will use the same key slot for the different
GTKs (there's no way for the driver to distinguish the different GTKs
from different AP VLAN interfaces). Thus, only the clients associated
to one AP VLAN interface (the one that was created last) can actually
use broadcast traffic.

Fix this by not programming any GTKs for AP VLAN interfaces into the hw
but fall back to using software crypto. The GTK for the underlying AP
interface is still sent to the driver.

That means, broadcast traffic to stations associated to an AP VLAN
interface is encrypted in software whereas broadcast traffic to
stations associated to the non-VLAN AP interface is encrypted in
hardware.

Cc: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: Helmut Schaa <helmut.schaa@googlemail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>