staging: comedi: pcl711: add AI cancel handler

Comedi subdevices that support asynchronous commands should have a
'cancel' handler to stop an in-progress command.  Add such a handler to
the pcl711 driver module.  I think merely setting the acquisition mode
to "software-triggered" would be sufficient, but also clear the
interrupt status for good luck.

Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Reviewed-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>