lantiq: fix lantiq-dsl output spam

lantiq: fix lantiq-dsl output spam

In case the adsl/vdsl service is disabled intentional, the output is
cluttered with the following message multiple times if the status of
the dsl line is queried:

  killall: vdsl_cpe_control: no process killed

If the dsl line status is queried by LuCI, the logfile is spammed with
the message.

Fix the issue by using killall in quite mode. In quite mode killall
still returns a non-zero value if the to be killed process can not be
found so that existing logic based on the return value still works.

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