staging: comedi: adv_pci1710: remove iorange member

The `iorange` member of `struct boardtype` is initialized but not used.
Get rid of it and the macro constants `IORANGE_171x` and `IORANGE_1720`.

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