staging: comedi: unioxx5: use comedi_alloc_spriv()

Use the helper function to allocate the memory and set the subdevice
private pointer. This allows the core to automatically free the
memory during the detach so the driver doesn't need to deal with it.

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>