staging: comedi: amplc_dio200: remove private data

The private data in this driver only has one member, 'intr_sd', which is
the index to the interrupt subdevice.

This member is initialized during the attach of the driver when the sd_intr
subdevice is detected in the boadinfo 'layout'. The member is then used in
the interrupt handler to get the pointer to the subdevice.

This member is not necessary. The comedi_device 'read_subdev' is also
initialized during the attach. This can be used in the interrupt handler
to get the subdevice pointer.

Refactor the code to not require the private data and remove the struct
and its 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>