serial: core: Use tty->index for port # in debug messages

The uart port may have already been removed by uart_remove_one_port();
use equivalent tty->index (which is always valid in these contexts)
instead.

Signed-off-by: Peter Hurley <peter@hurleysoftware.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>