staging: comedi: pcl711: define the mode register magic numbers

Define the magic numbers used with the mode register.

For the PCL711B board, the mode register also contains the irq number
that is being used. Introduce a helper function to write to the mode
register so that this irq number is always included when needed.

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>