staging: comedi: adv_pci_dio: simplify the 'detach'

Currently the 'detach' function in this driver walks thru the
boardinfo in order to find the subdevice index for any sdio
subdevices in order to call the subdev_8255_cleanup(). Then
it goes thru all the subdevices to clean the s->private pointer.

All the sdio subdevices are unique in that the s->type is
COMEDI_SUBD_DIO. Use that to know when to call the cleanup
for the 8255 subdevice.

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>