lantiq: create ATM/PTM interfaces with dsl as netdev name

lantiq: create ATM/PTM interfaces with dsl as netdev name

Renaming an atm etherbride using 'ip link' (via hotplug) is racy since the
original netdev might disappear before br2684ctl has finished appling it's
setting:

 local2.notice br2684ctl[1667]: Interface "nas0" created sucessfully
 local2.notice br2684ctl[1667]: Communicating over ATM 0.8.35, encapsulation: LLC
 kern.info kernel: dsl0: renamed from nas0
 kern.err kernel: br2684:br2684_regvcc: tried to attach to non-existent device
 local2.err br2684ctl[1667]: Could not configure interface:No such device or address

By passing the final used netdev name to br2684ctl_wrap another race
condition workaround will be enabled again.

Change the lantiq ptm driver to create a netdev with the name dsl as well.
Albeit the rename via 'ip link' works fine so far, using a different
approach for ptm then atm could be confusing.

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