lantiq: fix netdev setup in ltq-ptm-vdsl

The initial fix for 3.18 in r44346 wrongly treated the newly added
parameter of alloc_netdev to be another function pointer, causing
ether_config to be skipped during ptm netdev init.
Fix this by partially reverting r44346 and properly setting the newly
added macro parameter.
(Tested on VRX200 board)

A similar change might be needed for ltq-ptm-adsl as well.

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

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@44607 3c298f89-4303-0410-b956-a3cf2f4a3e73