mac80211: fix oops on mesh PS broadcast forwarding

Introduced with de74a1d9
"mac80211: fix WPA with VLAN on AP side with ps-sta".
Apparently overwrites the sdata pointer with non-valid data in
the case of mesh.
Fix this by checking for IFTYPE_AP_VLAN.

Signed-off-by: Marco Porsch <marco@cozybit.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>