staging: comedi: ni_at_ao: convert driver to use the comedi_8254 module

The hardware supported by this driver includes an 8254 timer. This timer is
not currently used, other than setting counters 1 and 2 to MODE4 to ensure
that the outputs are high.

For aesthetics, convert it to use the comedi_8254 module to provide support
for the 8254 timer. This will make it easier to add features later.

Signed-off-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Reviewed-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>