TTY: simserial, use tty_port_hangup

Convert shutdown to be tty_port_operations->shutdown. Then we can use
tty_port_hangup. (And we have to use tty_port_close.)

This means we no longer touch ASYNC_INITIALIZED, TTY_IO_ERROR. Also we
do not need to do any peculiar TTY logic in the file now.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Cc: Tony Luck <tony.luck@intel.com>
Cc: Fenghua Yu <fenghua.yu@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>