TTY: simserial, use tty_port_close_start

I.e. remove more copied bloat.

The only change is that we wait_until_sent now. Which is what we
really should do.

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>