are called whenever serio port is finishes being registered
       or unregistered. The callbacks are useful for drivers that
       share interrupt between several ports and there is a danger
       that interrupt handler will reference port that was just
       unregistered.
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>
Signed-off-by: Vojtech Pavlik <vojtech@suse.cz>