TTY: simserial, use tty_port_close_end

The code is identical except locking. But added locks to protect
counts do not hurt here. Rather the contrary.

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>