staging: comedi: ni_6527: use dev->read_subdev in interrupt handler

The comedi_subdevice in the interrupt handler is the dev->read_subdev
that was initialized during the attach. 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>