TTY: amiserial, use tty_port_close_start

Again, no need to duplicate the code. Let's use the helper.

Amiserial changes are only free of compilation errors. I have no
access to the hardware.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Cc: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>