staging: comedi: pcmmio: unwind the digital i/o subdevice init

The board supported by this driver only has one asic that provides the
two digital i/o subdevices. The first dio subdevice has 24 channels that
support interrupts. The second dio subdevice also has 24 channels but no
interrupt support.

To make the code clearer and easier to maintain, remove the for () loop
that initialized these subdevices.

Also, correctly initialize the interrupt subdevice. The current code does
not set the SDF_CMD_READ subdev_flag and does not set the dev->read_subdev
pointer in the comedi_device.

Use the dev->read_subdev pointer in the interrupt handler to get the
interrupt subdevice and private data pointer instead of searching for
them. To keep this patch reviewable, the extra indents in interrupt_pcmmio()
will be removed later.

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>