staging: comedi: adv_pci_dio: Add a counter subdevice to PCI-1751

The Advantech PCI-1751 has a 8254 counter chip on board.  Add it to the
device as a counter subdevice.  Apparently the counter can generate
interrupts although the driver does not currently use this capability.

Original patch by Ivan Russkih (Иван Русских) <vanekrus at gmail dot
com>.

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