lantiq: fix avm fritz box mac addresses

lantiq: fix avm fritz box mac addresses

It has been shown that the Fritz boxes have the correct mac address set
in the wireless calibration data/eeeprom. Use this mac address as base
for the ethernet and xdsl interface increment/decrement the address to
match the values stored in the tffs.

Signed-off-by: Mathias Kresin <dev@kresin.me>