TTY: fix DTR being raised on hang up

Make sure to check ASYNC_INITIALISED before raising DTR when waking up
from blocked open in tty_port_block_til_ready.

Currently DTR could get raised at hang up as a blocked process would
raise DTR unconditionally before checking for hang up and returning.

Signed-off-by: Johan Hovold <jhovold@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>