staging: comedi: adv_pci1710: tidy up PCI1720_* register defines

The PCI-1720 board is supported by this driver but uses a different
register map. For aesthetics, rename the defines to match the
PCI171X_* format.

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>