Staging: comedi: adv_pci_dio: Support Advantech PCI-1735U

Add support for the Advantech PCI-1735U card, including support for a
counter subdevice (based on an 82C54 counter timer chip).

The counter subdevice needs more testing, as the only person I know who
tried it couldn't get it to work!

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