staging: comedi: ii_pci20kc.c: break up the subdevice private data union

The union used for the subdevice private data just adds confusion.
Split the union into two separate private data structs.

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>