staging: comedi: pcmmio: remove subdevice private data

Only subdevice[2], the dio interrupt subdevice, uses the subdevice private data.
Move the members from struct pcmmio_subdev_private to struct pcmmio_private and
remove the subdevice private data. This also allows removing the allocation and
freeing of devpriv->sprivs.

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>