lantiq: move the dsl-modem config to network.@dsl-modem[-1]

make sure uci_defaults generates a proper uci config for the v/adsl modem

Signed-off-by: John Crispin <blogic@openwrt.org>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@40439 3c298f89-4303-0410-b956-a3cf2f4a3e73