lantiq: ltq-atm: set carrier status

This prevents the unconditionally start of pppd since netifd will be
triggered if the device goes into showtime. The same applies to lost
showtime as well.

In compare to the ptm driver, this changeset isn't strictly required,
since the "not in showtime" message is limited to the debug loglevel.
But it reduces the amount of ppp related messages significant.

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

SVN-Revision: 47916