staging: comedi: pcl711: use dev->read_subdev in interrupt handler

The comedi_subdevice used in the interrupt handler is the dev->read_subdev
that was initialized during the attach of the board. Use that instead of
accessing the dev->subdevices array directly.

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>