staging: comedi: pcl812: remove "spurious interrupt" noise

Because this driver is manually attached, the "spurious interrupt"
check in the interrupt handler should never happen. The interrupt is
only hooked up during the attach and it's released during the detach.

Leave the check but remove the noise. Also make sure to clear the
interrupt request in the hardware.

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>