staging: comedi: ii_pci20kc: use comedi_alloc_spriv()

For aesthetic reasons, use the helper function to allocate the
subdevice private data instead of hanging it on the device
private data. The core will free the memory during the detach.

Signed-off-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Cc: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>