staging: comedi: ni_65xx: remove the need for the subdevice private data

There is only one member in the subdevice private data, an unsigned value
that is the 'base_port' that the subdevice uses to access the port registers.

Just cast the appropriate value into s->private instead of allocating the
private data for each subdevice. The casts are a bit of a nusance but it
removes the unnecessary allocations.

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>