staging: comedi: adv_pci_dio: clarify subdevice index in pci1760_attach()

The subdevice index numbers are fixed when attaching to a pci1710 card.
Remove the subdev variable and just open code the array index numbers.

Signed-off-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>