[PPP] synchronous tty: convert dead_sem to completion

PPP synchronous tty channel driver: convert the semaphore dead_sem to a
completion

Signed-off-by: Matthias Kaehlcke <matthias.kaehlcke@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: David S. Miller <davem@davemloft.net>