staging: comedi: pcl711: save the irq for the pcl711b only when used

Move the storing of the irq in the mode register for the pcl711b board
so it's only done when the irq is actually used by the driver.

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>