lantiq: ltq-ptm: set carrier status

Principal purpose is to prevent the error message

  ifxmips_ptm_vdsl.c:281:ptm_hard_start_xmit: not in showtime

which is printed at least once per second to the serial console, if the
ptm interface is not in showtime, but a processes already sends
packages over that interface. This happens for adsl as well as vdsl
over ptm.

It's pppd which sends packages over the ptm device before in showtime.
As far as I can see, pppd is started unconditionally since netif can
not gather the link status of the ptm network interface.

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

SVN-Revision: 47917