lantiq: xway: rename nas0/ptm0 to dsl0

This change makes it possible to configure the wan/dsl ppp interface
settings independantly from the used TC-Layer (ATM/PTM).

By using dsl0 as interface name as for the xrx200 we can get rid of a
few conditionals which were introduced because of the different default
TC-Layer in xway and xrx200.

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