tty: use prepare/finish_wait

Use prepare_to_wait and finish_wait instead of add_wait_queue and
remove_wait_queue.

This avoids us setting a task state.

Signed-off-by: Jiri Slaby <jirislaby@gmail.com>
Signed-off-by: Alan Cox <alan@linux.intel.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>