staging: comedi: pcl711: fix pcl711_set_changain()

Currently the board->is_8112 handling in this function does not write
the value to the mux register. Also, the value for channels >= 8 is
calculated incorrectly.

Define the magic numbers used with the mux register and fix the function
so 8112 boards work correctly.

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>