staging: comedi: adv_pci_dio: simplify counter subdevice I/O

staging: comedi: adv_pci_dio: simplify counter subdevice I/O

Only two of the boards supported by this driver have an 8254 counter/timer.
Both of these boards have a single 8254 device. Currently the counter
subdevice functions are coded to support multiple 8254 devices. This is
unnecessary and just complicates the code.

Simplfy the subdevice functions to work for a single 8254 counter/timer and
refactor the driver (*attach) accordingly.

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>