mac80211: copy chandef from AP vif to VLANs

Instead of keeping track of all those special cases where
VLAN interfaces have no bss_conf.chandef, just make sure
they have the same as the AP interface they belong to.

Among others, this fixes a crash getting a VLAN's channel
from userspace since a NULL channel is returned as a good
result (return value 0) for VLANs since the commit below.

Cc: stable@vger.kernel.org [3.18 only]
Fixes: c12bc488 ("mac80211: return the vif's chandef in ieee80211_cfg_get_channel()")
Signed-off-by: Felix Fietkau <nbd@openwrt.org>
[rewrite commit log]
Signed-off-by: Johannes Berg <johannes.berg@intel.com>