gluon-core: lua: wireless: use libiwinfo-lua for find_phy()

find_phy() did not match OpenWrt's logic for PHY names anymore, and
could not handle PHYs that don't match the 'phyX' naming scheme. Instead
of trying to add more complex logic to our own code, simply do what
OpenWrt does and use iwinfo to resolve UCI sections to PHY names.