staging: comedi: rtd520: use DIV_ROUND_CLOSEST and DIV_ROUND_UP macros

Use the macros to clarify the 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>