staging: comedi: ni_labpc_common: use DIV_ROUND_CLOSEST to round divisor values

Use the DIV_ROUND_CLOSEST macro to clarify the (((x) + ((divisor) / 2)) / (divisor))
calculations.

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>