TTY: hvcs, use kref from tty_port

A simple switch. Except we convert destroy_hvcs_struct to be
tty_port_operations->destruct...

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Cc: linuxppc-dev@lists.ozlabs.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>