staging: comedi: amplc_dio200: add functions to access 8254 counters

Add our own functions to manipulate the '8254' counter chip instead of
the inline ones from "8253.h".  This will make the code less messy when
we add code to support new boards later.

Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>