USB: serial: fix tty-port initialized comments

Fix up the tty-port initialized comments which got truncated and
obfuscated when replacing the old ASYNCB_INITIALIZED flag.

Fixes: d41861ca

 ("tty: Replace ASYNC_INITIALIZED bit and update atomically")
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Johan Hovold <johan@kernel.org>