hostapd: improve 802.1x dynamic vlan support with bridge names

In r41872 and r42787 Dynamic VLAN support was reintroduced, but the vlan_bridge
parameter is not read while setting up the config, so the default is used which
is undesirable for some uses.

Signed-off-by: Ben Franske <ben.mm@franske.com>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@43473 3c298f89-4303-0410-b956-a3cf2f4a3e73