tty: move the async flags from the serial code into the tty includes

These are used with the tty_port flags which are tty generic so move the
flags into a more sensible place. This then makes it possible to add
helpers such as those suggested by Huang Shijie.

Signed-off-by: Alan Cox <alan@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>