luci-base: network.js: Show clients also from wifi VLANs.

Iterate through VLANs to get the clients connected on configured wifi
VLANs (as configured by `wifi-vlan` sections in `wireless` configuration).

This is a minimum support for VLANs on wireless network. The VLAN name is
nowhere displayed, but at least clients using VLANs are visible.

Signed-off-by: Oldřich Jedlička <oldium.pro@gmail.com>