mac80211: add support for wds_bridge hostapd feature

mac80211: add support for wds_bridge hostapd feature

hostapd allows putting WDS (4addr mode) clients into a separate bridge
other than the bridge regular (3addr mode) clients end up in. This is
useful for example giving WDS clients access to several VLANs
(trunking) while regular clients will end up inside a specific VLAN.

Add 'wds_bridge' config parameter for wifi-iface which contains the
name of the bridge. hostapd-mini already supports this feature, so all
needed is to add the UCI wrapping in mac80211.sh.

Signed-off-by: Daniel Golle <daniel@makrotopia.org>