staging: comedi: pcl726: enable the interrupt support code

Tidy up and enable the interrupt support code for the external trigger
source interrupt on the ACL-6126 board.

The interrupt handler is currently just a stub function. Once the async
command support is added this function will be completed.

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>