wifi: mac80211: fix decap offload for stations on AP_VLAN interfaces

Since AP_VLAN interfaces are not passed to the driver, check offload_flags
on the bss vif instead.

Reported-by: Howard Hsu <howard-yh.hsu@mediatek.com>
Fixes: 80a915ec ("mac80211: add rx decapsulation offload support")
Signed-off-by: Felix Fietkau <nbd@nbd.name>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>